Antacids.pdf

Pharmacotherapy of
Gastrointestinal disorders
By
Dr. Sairah Hafeez Kamran

Drugs Used in Acid‐Peptic Diseases
• Acid‐peptic diseases include gastroesophageal
reflux, peptic ulcer (gastric and duodenal), and
stress‐related mucosal injury. In all these
conditions, mucosal erosions or ulceration arise
when the caustic effects of aggressive factors
(acid, pepsin, bile) overwhelm the defensive
factors of the gastrointestinal mucosa (mucus and
bicarbonate secretion, prostaglandins, blood
flow, and the processes of restitution and
regeneration after cellular injury).

• Over 90% of peptic ulcers are caused by
infection with the bacterium Helicobacter
pylori or by use of nonsteroidal anti‐
inflammatory drugs (NSAIDs). Drugs used in
the treatment of acid‐peptic disorders may be
divided into two classes:
1. Agents that reduce intragastric acidity
2. Agents that promote mucosal defense.

Agents that Reduce Intragastric Acidity
• Physiology of Acid Secretion
• The parietal cell contains receptors for gastrin
(CCK‐B), histamine (H2), and acetylcholine
(muscarinic, M3). When acetylcholine (from vagal
postganglionic nerves) or gastrin (released from
antral G cells into the blood) bind to the parietal
cell receptors, they cause an increase in cytosolic
calcium, which in turn stimulates protein kinases
that stimulate acid secretion from a H+,K+ ATPase
(the proton pump) on the canalicular surface.

• In close proximity to the parietal cells are gut
endocrine cells called enterochromaffin‐like (ECL)
cells. ECL cells also have receptors for gastrin and
acetylcholine, which stimulate histamine release.
Histamine binds to the H2 receptor on the
parietal cell, resulting in activation of adenylyl
cyclase, which increases intracellular cyclic
adenosine monophosphate (cAMP) and activates
protein kinases that stimulate acid secretion by
the H+,K+ ATPase.

Antacids
• Antacids are weak bases that react with
gastric hydrochloric acid to form a salt and
water. Their principal mechanism of action is
reduction of intragastric acidity. After a meal,
approximately 45 mEq/h of hydrochloric acid
is secreted.

• A single dose of 156 mEq of antacid given 1
hour after a meal effectively neutralizes
gastric acid for up to 2 hours. However, the
acid‐neutralization capacity among different
proprietary formulations of antacids is highly
variable, depending on their rate of
dissolution (tablet versus liquid), water
solubility, rate of reaction with acid, and rate
of gastric emptying.

• Sodium bicarbonate (eg, baking soda, Alka
Seltzer) reacts rapidly with hydrochloric acid
(HCL) to produce carbon dioxide and sodium
chloride. Formation of carbon dioxide results
in gastric distention and belching.
• Unreacted alkali is readily absorbed,
potentially causing metabolic alkalosis when
given in high doses or to patients with renal
insufficiency.

• Sodium chloride absorption may exacerbate fluid
retention in patients with heart failure, hypertension,
and renal insufficiency.
• Calcium carbonate (40% calcium) is less soluble and
reacts more slowly than sodium bicarbonate with HCl
to form carbon dioxide and calcium chloride (CaCl2).
Like sodium bicarbonate, calcium carbonate may cause
belching or metabolic alkalosis.
• In achlorhydric patients, calcium carbonate should be
given with meals to increase absorption, or the patient
should be switched to calcium citrate (21% calcium),
which is somewhat better absorbed.

• Calcium carbonate is less soluble and reacts more
slowly than sodium bicarbonate with HCl to form
carbon dioxide and calcium chloride (CaCl2). Like
sodium bicarbonate, calcium carbonate may cause
belching or metabolic alkalosis. Calcium carbonate is
used for a number of other indications apart from its
antacid properties
• Excessive doses of either sodium bicarbonate or
calcium carbonate with calcium‐containing dairy
products can lead to hypercalcemia, renal insufficiency,
and metabolic alkalosis (milk‐alkali syndrome).

• About 15% of orally administered Ca2+ is
absorbed, causing a transient hypercalcemia.
Although this is not a problem in normal patients,
the hypercalcemia from as little as 3‐4 g of CaCO3
per day can be problematic in patients with
uremia. In the past, when large doses of NaHCO3
and CaCO3 were administered commonly with
milk or cream for the management of peptic
ulcer, the milk‐alkali syndrome (alkalosis,
hypercalcemia, and renal insufficiency) occurred
frequently

• Magnesium hydroxide or Aluminium
hydroxide react slowly with HCl to form
magnesium chloride or aluminium chloride
and water. Because no gas is generated,
belching does not occur. Metabolic alkalosis is
also uncommon because of the efficiency of
the neutralization reaction.

• Because unabsorbed magnesium salts may
cause an osmotic diarrhea and aluminum salts
may cause constipation, these agents are
commonly administered together in
proprietary formulations to minimize the
impact on bowel function. Both magnesium
and aluminum are absorbed and excreted by
the kidneys. Hence, patients with renal
insufficiency should not take these agents
long‐term

• Because unabsorbed magnesium salts may cause
an osmotic diarrhea and aluminum salts may
cause constipation, these agents are commonly
administered together in proprietary
formulations to minimize the impact on bowel
function.
• Both magnesium and aluminum are absorbed
and excreted by the kidneys. Hence, patients with
renal insufficiency should not take these agents
long‐term.

Contraindication
• All antacids may affect the absorption of other
medications by binding the drug (reducing its
absorption) or by increasing intragastric pH so
that the drug's dissolution or solubility
(especially weakly basic or acidic drugs) is
altered. Therefore, antacids should not be
given within 2 hours of doses of tetracyclines,
fluoroquinolones, itraconazole, and iron.

Mucosal Protective Agents
• Sucralfate
• Sucralfate is a salt of sucrose complexed to
sulfated aluminum hydroxide. In water or acidic
solutions it forms a viscous, tenacious paste that
binds selectively to ulcers or erosions for up to 6
hours.
• Sucralfate has limited solubility, breaking down
into sucrose sulfate (strongly negatively charged)
and an aluminum salt. Less than 3% of intact drug
and aluminum is absorbed from the intestinal
tract; the remainder is excreted in the feces.

• In an acid environment (pH <4), sucralfate undergoes
extensive cross‐linking to produce a viscous, sticky
polymer that adheres to epithelial cells and ulcers for
up to 6 hours after a single dose.
• In addition to inhibiting hydrolysis of mucosal proteins
by pepsin, sucralfate may have additional
cytoprotective effects, including stimulation of local
production of prostaglandins and epidermal growth
factor.
• Sucralfate also binds bile salts; thus some clinicians use
sucralfate to treat individuals with the syndromes of
biliary esophagitis or gastritis

• Sucralfate is administered in a dosage of 1 g four
times daily on an empty stomach (at least 1 hour
before meals). At present, its clinical uses are
limited. Sucralfate (administered as a slurry
through a nasogastric tube) reduces the
incidence of clinically significant upper
gastrointestinal bleeding in critically ill patients
hospitalized in the intensive care unit, although it
is slightly less effective than intravenous H2
antagonists.

• Sucralfate is still used by many clinicians for
prevention of stress‐related bleeding because
of concerns that acid inhibitory therapies
(antacids, H2 antagonists, and proton pump
inhibitors) may increase the risk of nosocomial
pneumonia

• Because it is not absorbed, sucralfate is
virtually devoid of systemic adverse effects.
Constipation occurs in 2% of patients due to
the aluminum salt. Because a small amount of
aluminum is absorbed, it should not be used
for prolonged periods in patients with renal
insufficiency. Sucralfate may bind to other
medications, impairing their absorption.

Prostaglandin Analogs: Misoprostol
• Misoprostol has both acid inhibitory and
mucosal protective properties. It stimulates
mucus and bicarbonate secretion and
enhance mucosal blood flow.

• It binds to a prostaglandin receptor (EP3
receptors) on parietal cells, reducing
histamine‐stimulated cAMP production and
causing modest acid inhibition. Prostaglandins
have a variety of other actions, including
stimulation of intestinal electrolyte and fluid
secretion, intestinal motility, and uterine
contractions.

• Misoprostol is rapidly absorbed after oral
administration and then is rapidly and
extensively de‐esterified to form misoprostol
acid, the principal and active metabolite of the
drug. Some of this conversion may occur in
the parietal cells. A single dose inhibits acid
production within 30 minutes; the therapeutic
effect peaks at 60‐90 minutes and lasts for up
to 3 hours.

• Food and antacids decrease the rate of
misoprostol absorption, resulting in delayed
and decreased peak plasma concentrations of
the active metabolite. The free acid is excreted
mainly in the urine, with an elimination t1/2 of
20‐40 minutes.

• Peptic ulcers develop in approximately 10–20% of
patients who receive long‐term NSAID therapy.
Misoprostol reduces the incidence of NSAID‐
induced ulcers to less than 3% and the incidence
of ulcer complications by 50%. It is approved for
prevention of NSAID‐induced ulcers in high‐risk
patients; however, misoprostol has never
achieved widespread use owing to its high
adverse‐effect profile and need for multiple daily
dosing.

• Diarrhea and cramping abdominal pain occur
in 10–20% of patients. Because misoprostol
stimulates uterine contractions ,it should not
be used during pregnancy

Bismuth Compounds
• Bismuth coats ulcers and erosions, creating a
protective layer against acid and pepsin. It
may also stimulate prostaglandin, mucus, and
bicarbonate secretion.
• Bismuth subsalicylate reduces stool frequency
and liquidity in acute infectious diarrhea, due
to salicylate inhibition of intestinal
prostaglandin and chloride secretion.

• Bismuth has direct antimicrobial effects and binds
enterotoxins, accounting for its benefit in
preventing and treating traveler's diarrhea.
• Bismuth compounds have direct antimicrobial
activity against H pylori.
• Bismuth compounds are widely used by patients
for the nonspecific treatment of dyspepsia and
acute diarrhea. Bismuth subsalicylate is also used
for the prevention of traveller's diarrhea.

• Side effects: Bismuth causes harmless
blackening of the stool, which may be
confused with gastrointestinal bleeding. Liquid
formulations may cause harmless darkening of
the tongue. Bismuth agents should be used
for short periods only and should be avoided
in patients with renal insufficiency.

• Two compounds are available
1. Bismuth subsalicylate
2. Bismuth subcitrate potassium.

H2‐Receptor Antagonists
• The H2 receptor antagonists inhibit acid
production by reversibly competing with
histamine for binding to H2 receptors on the
basolateral membrane of parietal cells. Four
different H2 receptor antagonists, which differ
mainly in their pharmacokinetics have been
identified

• Four H2 antagonists are in clinical use:
1. Cimetidine
2. Ranitidine
3. Famotidine
4. Nizatidine
• All four agents are rapidly absorbed from the
intestine. Cimetidine, ranitidine, and famotidine
undergo first‐pass hepatic metabolism resulting
in a bioavailability of approximately 50%.

Clinical Uses
• Gastroesophageal Reflux Disease (GERD)
• H2 antagonists may be taken prophylactically before
meals in an effort to reduce the likelihood of
heartburn. Frequent heartburn is better treated with
twice‐daily H2 antagonists.
• Peptic Ulcer Disease
• Nocturnal acid suppression by H2 antagonists is
effective for ulcer healing in most patients with
uncomplicated gastric and duodenal ulcers. Hence, all
the agents may be administered once daily at bedtime,
resulting in ulcer healing rates of more than 80–90%
after 6–8 weeks of therapy.

• H2 antagonists may be given daily at bedtime
in half of the usual ulcer therapeutic dose to
prevent ulcer recurrence (eg, ranitidine, 150
mg; famotidine, 20 mg).
• Nonulcer Dyspepsia
• H2 antagonists are commonly used as over‐
the‐counter agents and prescription agents for
treatment of intermittent dyspepsia not
caused by peptic ulcer.

• Prevention of Bleeding from Stress‐Related
Gastritis
• Intravenous H2 antagonists are preferable over
intravenous proton pump inhibitors because
of their proven efficacy and lower cost.
Continuous infusions of H2 antagonists are
generally preferred to bolus infusions because
they achieve more consistent, sustained
elevation of intragastric pH.

Adverse Effects
• H2 antagonists are extremely safe drugs.
Adverse effects occur in less than 3% of
patients and include diarrhea, headache,
fatigue, myalgias, and constipation. Some
studies suggest that intravenous H2
antagonists (or proton pump inhibitors) may
increase the risk of nosocomial pneumonia in
critically ill patients.

• Cimetidine inhibits binding of
dihydrotestosterone to androgen receptors,
inhibits metabolism of estradiol, and increases
serum prolactin levels.
• When used long‐term or in high doses, it may
cause gynecomastia or impotence in men and
galactorrhea in women. These effects are
specific to cimetidine and do not occur with
the other H2 antagonists.

• Mental status changes (confusion,
hallucinations, agitation) may occur with
administration of intravenous H2 antagonists,
especially in patients in the intensive care unit
who are elderly or who have renal or hepatic
dysfunction. These events may be more
common with cimetidine. Mental status
changes rarely occur in ambulatory patients.

• H2 antagonists may rarely cause blood
dyscrasias. Blockade of cardiac H2 receptors
may cause bradycardia. Rapid intravenous
infusion may cause bradycardia and
hypotension through blockade of cardiac H2
receptors; therefore, intravenous injections
should be given over 30 minutes. H2
antagonists rarely cause reversible
abnormalities in liver chemistry.

Drug Interactions
• Cimetidine inhibits important hepatic cytochrome
P450 drug metabolism pathways, including those
catalyzed by CYP1A2, CYP2C9, CYP2D6, and
CYP3A4. H2 antagonists compete with creatinine
and certain drugs (eg, procainamide) for renal
tubular secretion. All of these agents except
famotidine inhibit gastric first‐pass metabolism of
ethanol, especially in women. Increased
bioavailability of ethanol could lead to increased
blood ethanol levels.

Proton Pump Inhibitors
• Five proton pump inhibitors are available for
clinical use: omeprazole, lansoprazole,
rabeprazole, pantoprazole, and
esomeprazole. All are substituted
benzimidazoles that resemble H2 antagonists
in structure but have a completely different
mechanism of action.

• Proton pump inhibitors are administered as
inactive prodrugs. To protect the acid‐labile
prodrug from rapid destruction within the
gastric lumen, oral products are formulated
for delayed release as acid‐resistant, enteric‐
coated capsules or tablets. After passing
through the stomach into the alkaline
intestinal lumen, the enteric coatings dissolve
and the prodrug is absorbed.

Mechanism of action
• The proton pump inhibitors are lipophilic weak
bases (pKa 4–5) and after intestinal absorption
diffuse readily across lipid membranes into
acidified compartments (eg, the parietal cell
canaliculus). The prodrug rapidly becomes
protonated within the canaliculus and is
concentrated more than 1000‐fold by Henderson‐
Hasselbalch trapping. There, it rapidly undergoes
a molecular conversion to the active form, a
reactive cation, which forms a covalent disulfide
bond with the H+‐K+ ATPase, irreversibly
inactivating the enzyme.

Pharmacokinetics
• The bioavailability of all agents is decreased
approximately 50% by food; hence, the drugs
should be administered on an empty stomach. In
a fasting state, only 10% of proton pumps are
actively secreting acid and susceptible to
inhibition. Proton pump inhibitors should be
administered approximately 1 hour before a meal
(usually breakfast), so that the peak serum
concentration coincides with the maximal activity
of proton pump secretion.

• The drugs have a short serum half‐life of about
1.5 hours, but acid inhibition lasts up to 24 hours
owing to the irreversible inactivation of the
proton pump. At least 18 hours are required for
synthesis of new H+,K+ ATPase pump molecules.
Because not all proton pumps are inactivated
with the first dose of medication, up to 3–4 days
of daily medication are required before the full
acid‐inhibiting potential is reached. Similarly,
after stopping the drug, it takes 3–4 days for full
acid secretion to return.

Clinical uses
• Gastroesophageal Reflux Disease (GERD)
• Proton pump inhibitors are the most effective
agents for the treatment of nonerosive and
erosive reflux disease, esophageal
complications of reflux disease. Once‐daily
dosing provides effective symptom relief and
tissue healing in 85–90% of patients; up to
15% of patients require twice‐daily dosing.

• Peptic Ulcer Disease
• Proton pump inhibitors cause more rapid
symptom relief and faster ulcer healing for
duodenal ulcers and, to a lesser extent, gastric
ulcers. All the pump inhibitors heal more than
90% of duodenal ulcers within 4 weeks and a
similar percentage of gastric ulcers within 6–8
weeks.

• For H pylori‐associated ulcers, there are two
therapeutic goals: to heal the ulcer and to
eradicate the organism. The most effective
regimens for H pylori eradication are
combinations of two antibiotics and a proton
pump inhibitor. Proton pump inhibitors promote
eradication of H pylori through several
mechanisms: direct antimicrobial properties
(minor) and—by raising intragastric pH—lowering
the minimal inhibitory concentrations of
antibiotics against H pylori.

• The best treatment regimen consists of a 14‐day
regimen of "triple therapy": a proton pump
inhibitor twice daily; clarithromycin, 500 mg
twice daily; and either amoxicillin, 1 g twice daily,
or metronidazole, 500 mg twice daily.
• After completion of triple therapy, the proton
pump inhibitor should be continued once daily
for a total of 4–6 weeks to ensure complete ulcer
healing.

• NSAID‐Associated Ulcers
• In patients with NSAID‐induced ulcers who
require continued NSAID therapy, treatment
with a once‐ or twice‐daily proton pump
inhibitor more reliably promotes ulcer healing.
• Proton pump inhibitors taken once daily are
effective in reducing the incidence of ulcers
and ulcer complications in patients taking
aspirin or other NSAIDs.

• Prevention of Rebleeding from Peptic Ulcers
• Rebleeding of this subset of high‐risk ulcers is
reduced significantly with proton pump
inhibitors administered for 3–5 days either as
high‐dose oral therapy (eg, omeprazole, 40 mg
orally twice daily) or as a continuous
intravenous infusion.

• Nonulcer Dyspepsia
• Proton pump inhibitors have modest efficacy for
treatment of nonulcer dyspepsia, benefiting 10–20%
more patients than placebo.
• Prevention of Stress‐Related Mucosal Bleeding
• Proton pump inhibitors (given orally, by nasogastric
tube, or by intravenous infusions) may be administered
to reduce the risk of clinically significant stress‐related
mucosal bleeding in critically ill patients. The only
proton pump inhibitor approved by the Food and Drug
Administration (FDA) for this indication is an oral
immediate‐release omeprazole formulation

• Gastrinoma and Other Hypersecretory
Conditions
• Patients with isolated gastrinomas are best
treated with surgical resection. In patients with
metastatic or unresectable gastrinomas, massive
acid hypersecretion results in peptic ulceration,
erosive esophagitis, and malabsorption.
• With proton pump inhibitors, excellent acid
suppression can be achieved in all patients.
Typical doses of omeprazole are 60–120 mg/d.

Adverse Effects
• Proton pump inhibitors are extremely safe.
Diarrhea, headache, and abdominal pain are
reported in 1–5% of patients.
• Acid is important in releasing vitamin B12
from food. A minor reduction in oral
cyanocobalamin absorption occurs during
proton pump inhibition, potentially leading to
subnormal B12 levels with prolonged therapy.

• A small increased risk of enteric infections
may exist in patients taking proton pump
inhibitors.
• The rise in serum gastrin levels in patients
receiving long‐term therapy with proton pump
inhibitors raises a theoretical concern because
gastrin may stimulate hyperplasia of ECL cells.

Muscarinic antagonist
• The M1 muscarinic receptor antagonists
pirenzepine and telenzepine can reduce basal
acid production by 40‐50% and long have
been used to treat patients with peptic ulcer
disease in countries other than the U.S. The
ACh receptor on the parietal cell itself is of the
M3 subtype, and these drugs are believed to
suppress neural stimulation of acid production
via actions on M1 receptors on ganglia

• Rebamipide s used for ulcer therapy in parts of Asia. It
appears to exert a cytoprotective effect both by
increasing prostaglandin generation in gastric mucosa
and by scavenging reactive oxygen species.
• Ecabet appears to increase the formation of PGE2 and
PGI2, also is used for ulcer therapy, mostly in Japan.
• Carbenoxolone, a derivative of glycyrrhizic acid found
in licorice root, has been used with modest success for
ulcer therapy in Europe. Its exact mechanism of action
is not clear, but it may alter the composition and
quantity of mucin.

Emesis
• The act of emesis and the sensation of nausea
that accompanies it generally are the protective
reflexes that serve to rid the stomach and
intestine of toxic substances and prevent their
further ingestion.
• Vomiting is regulated centrally by the vomiting
centre and the chemoreceptor trigger zone (CTZ),
both of which lie in the medulla. The CTZ is
sensitive to chemical stimuli and is the main site
of action of many emetic and antiemetic drugs.

• The blood‐brain barrier in the neighbourhood of the
CTZ is relatively permeable, allowing circulating
mediators to act directly on this centre. The CTZ also
regulates motion sickness. Impulses from the CTZ pass
to those areas of the brain stem‐known collectively as
the vomiting centre‐that control and integrate the
visceral and somatic functions involved in vomiting.
• Retching is the anti‐peristalsis of the stomach and
esophagus without expelling any vomitus. It can be
caused by foul odours or withdrawal of some
medications after vomiting stops. Retching is a part of
vomiting process.

• Vomiting is a complex process that consists of a
pre‐ejection phase (gastric relaxation and
retroperistalsis), retching (rhythmic action of
respiratory muscles preceding vomiting and
consisting of contraction of abdominal and
intercostal muscles and diaphragm against a
closed glottis), and ejection (intense contraction
of the abdominal muscles and relaxation of the
upper esophageal sphincter). This is accompanied
by multiple autonomic phenomena including
salivation, shivering, and vasomotor changes.

• The process appears to be coordinated by a
central emesis center in the lateral reticular
formation of the mid‐brainstem adjacent to both
the chemoreceptor trigger zone (CTZ) in the area
postrema (AP) at the bottom of the fourth
ventricle and the solitary tract nucleus (STN). The
lack of a blood‐brain barrier allows the CTZ to
monitor blood and cerebrospinal fluid constantly
for toxic substances and to relay information to
the emesis center to trigger nausea and vomiting.

• The emesis center also receives information
from the gut, principally by the vagus nerve
(via the STN) but also by splanchnic afferents
via the spinal cord. Two other important
inputs to the emesis center come from the
cerebral cortex (particularly in anticipatory
nausea or vomiting) and the vestibular
apparatus (in motion sickness).

• The vestibular system is important in motion sickness
via cranial nerve VIII (vestibulocochlear nerve). It is rich
in muscarinic M1 and histamine H1 receptors.
• Vagal and spinal afferent nerves from the
gastrointestinal tract are rich in 5‐HT3 receptors.
Irritation of the gastrointestinal mucosa by
chemotherapy, radiation therapy, distention, or acute
infectious gastroenteritis leads to release of mucosal
serotonin and activation of these receptors, which
stimulate vagal afferent input to the vomiting center
and chemoreceptor trigger zone

• In turn, the center sends out efferents to the
nuclei responsible for respiratory, salivary, and
vasomotor activity, as well as to striated and
smooth muscle involved in the act. The CTZ has
high concentrations of receptors for serotonin (5‐
HT3), dopamine (D2), and opioids; the STN is rich
in receptors for enkephalin, histamine, and ACh,
and also contains 5‐HT3 receptors. A variety of
these neurotransmitters are involved in nausea
and vomiting

• Nausea and vomiting may be manifestations
of a wide variety of conditions, including
adverse effects from medications; systemic
disorders or infections; pregnancy; vestibular
dysfunction; central nervous system infection
or increased pressure; peritonitis;
hepatobiliary disorders; radiation or
chemotherapy; and gastrointestinal
obstruction, dysmotility, or infections.

Emetics
There are two types of emetics
• Centrally acting emetics for e.g apomorphine
• Reflex emetics for e.g Ipecachuana, metallic
salts like copper sulfate and sodium chloride

Apomorphine
• It is centrally acting emetic. It exclusively acts on CTZ. In
man 1 mg of apomorphine given subcutaneously usually
induces emesis within 15‐ 30 minutes. If the dose is given
below individual’s threshold, it may produce feeling of
nausea and vomiting and feeling of lethargy without
vomiting, but a dose that is substantially supralaminal may
also fail to induce emesis. This effect is a reflection of drug’s
ability to depress the emetic center.
• The emetic effect of apomporphine is also due to its action
as agonist on the dopamine receptors.
• Emesis has been used in aversion therapy to break the
pattern of drug use in drug addicts or alcohol abusers.

• Apomorohine is administered at the same time
when alcohol is offered. Violent vomiting follows
and after some period, a conditioned response is
developed so that when alcohol alone is
administered, nausea and vomiting develops. In
this way it is expected that aversion therapy will
prevent the addicts to return to drinking alcohol.
• Apomorphine is also used to induce emesis in
animal models employed for studying the
mechanism of action of anti emetics.

Reflex emetics
• Copper sulfate (CuSO4) has been advocated as
a rapidly acting emetic for use in the
immediate treatment of poisoning by
ingestion. There is, however, a high potential
for systemic absorption of toxic amounts of
copper, especially when emesis does not
occur.

• Syrup of ipecac is an over‐the‐counter
preparation that contains emetine, a toxic
alkaloid that produces vomiting by acting as a
stomach irritant. It usually, but not
consistently, produces vomiting in 15–30 min.
If repeated use fails to induce emesis, then
gastric lavage is necessary to remove the
emetine to prevent additional toxicosis.

• Hypertonic solutions of sodium chloride
reflexly induce emesis but they may cause
problems like hypernatremia.
• Many drugs induce emesis by irritating GIT for
example tetracyclines, colchicine and
antineoplastic drugs.
• Ergot alkaloids stimulate CTZ by acting on
dopamine receptors

• Chloropicrin has been used as fumigant. In
World War 1 German forces used
concentrated chloropicrin against Allied forces
as a tear gas. While not as lethal as other
chemical weapons, it caused vomiting and
forced Allied soldiers to remove their masks to
vomit, exposing them to other, more toxic
chemical gases used as weapons during the
war.

Anti emetics
• Several antiemetic agents are available, and
these are generally used for specific
conditions, although there may be some
overlap. Such drugs are of particular
importance as an adjunct to cancer
chemotherapy, where the nausea and
vomiting produced by many cytotoxic drugs is
almost unendurable.

Serotonin 5‐HT3 Antagonists
• Ondansetron, Granisetron, Dolasetron, and
Palonosetron
• Mechanism of action
• Selective 5‐HT3‐receptor antagonists have potent
antiemetic properties that are mediated in part
through central 5‐HT3‐receptor blockade in the
vomiting center and chemoreceptor trigger zone
but mainly through blockade of peripheral 5‐HT3
receptors on extrinsic intestinal vagal and spinal
afferent nerves.

• The antiemetic action of these agents is
restricted to emesis attributable to vagal
stimulation (eg, postoperative) and
chemotherapy; other emetic stimuli such as
motion sickness are poorly controlled.

Clinical use
• Chemotherapy‐Induced Nausea and Vomiting
• 5‐HT3‐receptor antagonists are the primary agents for
the prevention of acute chemotherapy‐induced nausea
and emesis. When used alone, these drugs have little
or no efficacy for the prevention of delayed nausea and
vomiting (ie, occurring > 24 hours after chemotherapy).
The drugs are most effective when given as a single
dose by intravenous injection 30 minutes prior to
administration of chemotherapy in the following doses:
ondansetron, 8 mg or 0.15 mg/kg; granisetron, 1 mg;
dolasetron, 100 mg; or palonosetron, 0.25 mg.

• Postoperative and Postradiation Nausea and
Vomiting
• 5‐HT3‐receptor antagonists are used to prevent or
treat postoperative nausea and vomiting.
Because of adverse effects and increased
restrictions on the use of other antiemetic
agents, 5‐HT3‐receptor antagonists are
increasingly used for this indication. They are also
effective in the prevention and treatment of
nausea and vomiting in patients undergoing
radiation therapy to the whole body or abdomen.

• Adverse Effects
• The 5‐HT3‐receptor antagonists are well‐
tolerated agents with excellent safety profiles.
The most commonly reported adverse effects
are headache, dizziness, and constipation. All
four agents cause a small but significant
prolongation of the QT interval.

Corticosteroids
• Corticosteroids (dexamethasone,
methylprednisolone) have antiemetic properties.
These agents appear to enhance the efficacy of 5‐
HT3‐receptor antagonists for prevention of acute
and delayed nausea and vomiting in patients
receiving moderately to highly emetogenic
chemotherapy regimens. Although a number of
corticosteroids have been used, dexamethasone,
8–20 mg intravenously before chemotherapy,
followed by 8 mg/d orally for 2–4 days, is
commonly administered.

Phenothiazines & Butyrophenones
• Phenothiazines are antipsychotic agents that
can be used for their potent antiemetic and
sedative properties. The antiemetic properties
of phenothiazines are mediated through
inhibition of dopamine and muscarinic
receptors. Sedative properties are due to their
antihistamine activity. The agents most
commonly used as antiemetics are
prochlorperazine, promethazine, and
thiethylperazine.

• Antipsychotic butyrophenones also possess
antiemetic properties due to their central
dopaminergic blockade. The main agent used is
droperidol, which can be given by intramuscular
or intravenous injection. In antiemetic doses,
droperidol is extremely sedating. Until recently, it
was used extensively for postoperative nausea
and vomiting, in conjunction with opiates and
benzodiazepines for sedation for surgical and
endoscopic procedures, for neuroleptanalgesia,
and for induction and maintenance of general
anesthesia.

• Extrapyramidal effects and hypotension may
occur. Droperidol results in fatal episodes of
ventricular tachycardia. Therefore, droperidol
should not be used in patients with QT
prolongation and should be used only in
patients who have not responded adequately
to alternative agents.

Substituted Benzamides
• Substituted benzamides include
metoclopramide and trimethobenzamide.
Their primary mechanism of antiemetic action
is believed to be dopamine‐receptor (D2)
blockade. Trimethobenzamide also has weak
antihistaminic activity. The antiemetic action
of high doses of metoclopramide in nausea
and vomiting induced by anti‐neoplastic drugs
is thought to be due to antagonism of 5HT3
receptors as well.

• For prevention and treatment of nausea and vomiting,
metoclopramide may be given in the relatively high
dosage of 10–20 mg orally or intravenously every 6
hours. 10 – 15 mg given prior to meals at bed time is
helpful for diabetic gastroparesis.
• The usual dose of trimethobenzamide is 250 mg orally,
200 mg rectally, or 200 mg by intramuscular injection.
The principal adverse effects of these central dopamine
antagonists are extrapyramidal: restlessness, dystonias,
and parkinsonian symptoms.

• Metoclopramide enhances the motility of
stomach, pylorus and small intestine. This action
is not due to central stimulation of vagus nerve
but it is due to peripheral stimulation of GIT and
this action is blocked by atropine.
• It is used to control nausea and vomiting that
occurs in uremia, GIT cancer, radiation sickness,
gastritis, and peptic ulcer. The drug is in effective
in motion sickness and vomiting occuring in
labryinth disorders.

Anticholinergic Agents
• The most commonly used muscarinic receptor
antagonist is scopolamine (hyoscine), which
can be injected as the hydrobromide, but
usually is administered as the free base in the
form of a transdermal patch. Its principal
utility is in the prevention and treatment of
motion sickness, although it has been shown
to have some activity in postoperative nausea
and vomiting tool.

• In general, anticholinergic agents have no role
in chemotherapy‐induced nausea. Dry mouth
and blurred vision are the most common
unwanted effects. Drowsiness also occurs, but
the drug has less sedative action than the
antihistamines because of poor central
nervous system penetration.

Cannabinoids
• Dronabinol (‐9‐tetrahydrocannabinol) is a
naturally occurring cannabinoid that can be
synthesized chemically or extracted from the
marijuana plant, Cannabis sativa. The
mechanism of the anti‐emetic action of
dronabinol relates to stimulation of the CB1
subtype of cannabinoid receptors on neurons
in and around the vomiting center in the
brainstem

• Dronabinol is a highly lipid‐soluble compound
that is absorbed readily after oral
administration; its onset of action occurs
within an hour, and peak levels are achieved
within 2‐4 hours. It undergoes extensive first‐
pass metabolism with limited systemic
bioavailability after single doses. Active and
inactive metabolites are formed in the liver;
the principal active metabolite is 11‐OH‐delta‐
9‐tetrahydrocannabinol.

• These metabolites are excreted primarily via the
biliary‐fecal route, with only 10‐15% excreted in
the urine.
• Dronabinol is a useful prophylactic agent in
patients receiving cancer chemotherapy when
other anti‐emetic medications are not effective. It
also can stimulate appetite and has been used in
patients with acquired immunodeficiency
syndrome (AIDS) and anorexia.

• Dronabinol has complex effects on the CNS,
including a prominent central sympathomimetic
activity. This can lead to palpitations, tachycardia,
vasodilation, hypotension. Patient supervision is
necessary because marijuana CNS effects like
euphoria, somnolence, detachment, dizziness,
anxiety, nervousness, panic, etc. can occur, as can
more disturbing effects such as paranoid
reactions and thinking abnormalities. After
abrupt withdrawal of dronabinol, an abstinence
syndrome (irritability, insomnia, and restlessness)
can occur.

• Nabilone is a synthetic cannabinoid with a mode
of action similar to that of dronabinol. Nabilone is
a useful prophylactic agent in patients receiving
cancer chemotherapy when other anti‐emetic
medications are not effective. A dose (1‐2 mg)
can be given the night before chemotherapy. The
adverse effects are largely the same as for
dronabinol, with significant CNS actions in >10%
of patients. Cardiovascular, GI, and other side
effects are also common and together with the
CNS actions limit the usefulness of this agent.

Benzodiazepines
• Benzodiazepines, such as lorazepam and
alprazolam, by themselves are not very
effective anti‐emetics, but their sedative,
amnesic, and anti‐anxiety effects can be
helpful in reducing the anticipatory
component of nausea and vomiting in
patients.

Neurokinin Receptor Antagonists
• Neurokinin 1 (NK1)‐receptor antagonists have
antiemetic properties that are mediated through
central blockade in the area postrema.
Aprepitant (an oral formulation) is a highly
selective NK1‐receptor antagonist that crosses the
blood‐brain barrier and occupies brain NK1
receptors. It has no affinity for serotonin,
dopamine, or corticosteroid receptors.
Fosaprepitant is an intravenous formulation that
is converted within 30 minutes after infusion to
aprepitant.

Constipation
• Scientific definitions of constipation rely
mostly on stool number. In the absence of
medication that might stimulate or inhibit
intestinal movements, the frequency of
defecation mainly depends on nature of diet
and on previous habit or training.
• Most surveys have found the normal stool
frequency on a Western diet to be at least
three times a week

• Patients use the term constipation not only for
decreased frequency, but also for difficulty in
initiation or passage, passage of firm or small‐
volume feces, or a feeling of incomplete
evacuation.

• Constipation has many reversible or secondary causes, including
1. Lack of dietary fiber,
2. Action of drugs
3. Action of toxins
4. Hormonal disturbances
5. Neurogenic disorders
6. Systemic illnesses.
7. Inhibition of defecation reflex by pain arising from (a)
Hemorrhoids, (b) ulceration of anal mucosa, (c) surgical or
traumatic wounds of anus.
8. Pregnancy
9. Acute illness (prolonged bed rest)

• In most cases of chronic constipation, no specific
cause is found. Up to 60% of patients presenting
with constipation have normal colonic transit.
• Colonic motility is responsible for mixing luminal
contents to promote absorption of water and
moving them from proximal to distal segments by
means of propulsive contractions. Mixing in the
colon is accomplished in a way similar to that in
the small bowel: by short‐ or long‐duration,
stationary (nonpropulsive) contractions.

• Propulsive contractions in the colon include giant
migrating contractions, also known as colonic
mass actions or mass movements, which
propagate caudally over extended lengths in the
colon and evoke mass transfer of feces from the
right to the left colon once or twice a day.
Disturbances in motility therefore may have
complex effects on bowel movements.
"Decreased motility" of the mass action type and
"increased motility" of the nonpropulsive type
may lead to constipation.

Laxatives and Purgatives
• The terms laxatives, cathartics, purgatives,
aperients, and evacuants often are used
interchangeably. There is a distinction,
however, between laxation (the evacuation of
formed fecal material from the rectum) and
catharsis (the evacuation of unformed, usually
watery fecal material from the entire colon).
Most of the commonly used agents promote
laxation, but some are actually cathartics that
act as laxatives at low doses.

• Aperient is taken from a latin word ‘aperire’ which means
‘to open’.
• Lenitive is taken from ‘lenire’ which means ‘to soften’.
• Laxative from ‘laxare’ means ‘to loosen’
• Evacuative from ‘vacurae’ means ‘to empty’
• Purgative from ‘purgare’ means ‘to purify’
• Cathartic is taken from Greek word ‘Kathartikas’ which
means to utterly clean
• This can be arranged in order of increasing severity or
potency
• aperient< lenitive< laxative< evacuative< purgative<
cathartic.

• Laxatives generally act in one of the following
ways:
1. enhancing retention of intraluminal fluid by
hydrophilic or osmotic mechanisms;
2. decreasing net absorption of fluid by effects on
small‐ and large‐bowel fluid and electrolyte
transport
3. altering motility by either inhibiting segmenting
(nonpropulsive) contractions or stimulating
propulsive contractions.

Classification of Laxatives
• Luminally active agents
1. Hydrophilic colloids; bulk‐forming agents
(bran, psyllium, etc.)
2. Osmotic agents (non‐absorbable inorganic
salts or sugars)
3. Stool‐wetting agents (surfactants) and
emollients (docusate, mineral oil)

• Nonspecific stimulants or irritants (with effects
on fluid secretion and motility)
1. Diphenylmethanes (bisacodyl)
2. Anthraquinones (senna and cascara)
3. Castor oil
• Prokinetic agents (acting primarily on motility)
1. 5‐HT4 receptor agonists

• Dopamine receptor antagonists
• Motilides (erythromycin)

Classification and Comparison of
Representative Laxatives
• SOFTENING OF FECES, 1‐3 DAYS
• Bulk‐forming laxatives
1. Bran
2. Psyllium preparations
3. Methylcellulose
4. Calcium polycarbophil
• Surfactant laxatives
1. Docusates
2. Poloxamers
3. Lactulose

• SOFT OR SEMIFLUID STOOL, 6‐8 HOURS
• Stimulant laxatives
1. Diphenylmethane derivatives
2. Bisacodyl
• Anthraquinone derivatives
1. Senna
2. Cascara sagrada

• WATERY EVACUATION, 1‐3 HOURS
• Osmotic laxatives
1. Sodium phosphates
2. Magnesium sulfate
3. Milk of magnesia
4. Magnesium citrate
5. Castor oil

Dietary Fiber and Supplements
• The bulk, softness, and hydration of feces depend
on the fiber content of the diet. Fiber is defined
as that part of food that resists enzymatic
digestion and reaches the colon largely
unchanged. Colonic bacteria ferment fiber to
varying degrees, depending on its chemical
nature and water solubility. Fermentation of fiber
has two important effects:
• it produces short‐chain fatty acids that are
trophic for colonic epithelium
• it increases bacterial mass

• TYPES OF FIBER
• Nonpolysaccharides
1. Lignin
2. Cellulose
• Noncellulose polysaccharides
1. Hemicellulose
2. Mucilages and gums
3. Pectins

Bulk forming laxatives
• Bran, the residue left when flour is made from
cereal grains, contains >40% dietary fiber. Wheat
bran, with its high lignin content, is most effective
at increasing stool weight. Fruits and vegetables
contain more pectins and hemicelluloses, which
are more readily fermentable and produce less
effect on stool transit. Psyllium husk, derived
from the seed of the plantago herb (Plantago
ovata; known as ispaghula or isabgol in many
parts of the world), is a component of many
commercial products for constipation

• Psyllium husk contains a hydrophilic mucilloid that
undergoes significant fermentation in the colon, leading to
an increase in colonic bacterial mass. The usual dose is 2.5‐
4 g (1‐3 teaspoonfuls in 250 mL of fruit juice), titrated
upward until the desired goal is reached.
• A variety of semisynthetic celluloses—e.g., methylcellulose
and the hydrophilic resin calcium polycarbophil, a polymer
of acrylic acid resin—also are available. These poorly
fermentable compounds absorb water and increase fecal
bulk.
• Malt soup extract, an extract of malt from barley grains
that contains small amounts of polymeric carbohydrates,
proteins, electrolytes, and vitamins, is another orally
administered bulk‐forming agent.

• Fiber is contraindicated in patients with
obstructive symptoms and in those with
megacolon or megarectum. Fecal impaction
should be treated before initiating fiber
supplementation. Bloating is the most common
side effect of soluble fiber products (perhaps due
to colonic fermentation), but it usually decreases
with time. Calcium polycarbophil preparations
release Ca2+ in the GI tract and thus should be
avoided by patients who must restrict their intake
of calcium or who are taking tetracycline.

Osmotically Active Agents
• Saline Laxatives
• Laxatives containing magnesium cations or
phosphate anions commonly are called saline
laxatives: magnesium sulfate, magnesium
hydroxide, magnesium citrate, sodium
phosphate. Their cathartic action is believed to
result from osmotically mediated water
retention, which then stimulates peristalsis.
Other mechanisms may contribute to their
effects, including the production of inflammatory
mediators.

• Magnesium‐containing laxatives may stimulate
the release of cholecystokinin, which leads to
intraluminal fluid and electrolyte accumulation
and to increased intestinal motility. The intensely
bitter taste of some preparations may induce
nausea and can be masked with citrus juices.
• Phosphate salts are better absorbed than
magnesium‐based agents and therefore need to
be given in larger doses to induce catharsis.
Phosphates induce nephropathy.

• Magnesium‐ and phosphate‐containing
preparations must be used with caution or
avoided in patients with renal insufficiency,
cardiac disease, or preexisting electrolyte
abnormalities, and in patients on diuretic
therapy. Patients taking >45 mL of oral sodium
phosphate as a prescribed bowel preparation
may experience electrolyte shifts that pose a risk
for the development of symptomatic
dehydration, renal failure, metabolic acidosis,
tetany from hypocalcemia, and even death in
vulnerable populations.

• Nondigestible Sugars and Alcohols
• Lactulose is a synthetic disaccharide of galactose and
fructose that resists intestinal disaccharidase activity.
This and other non‐absorbable sugars such as sorbitol
and mannitol are hydrolyzed in the colon to short‐chain
fatty acids, which stimulate colonic propulsive motility
by osmotically drawing water into the lumen. Sorbitol
and lactulose are equally efficacious in the treatment
of constipation caused by opioids and vincristine, of
constipation in the elderly, and of idiopathic chronic
constipation.

• They are available as 70% solutions, which are
given in doses of 15‐30 mL at night, with
increases as needed up to 60 mL per day in
divided doses. Effects may not be seen for 24‐
48 hours after dosing is begun. Abdominal
discomfort or distention and flatulence are
relatively common in the first few days of
treatment but usually subside with continued
administration.

• Lactulose also is used to treat hepatic
encephalopathy. Patients with severe liver
disease have an impaired capacity to detoxify
ammonia coming from the colon, where it is
produced by bacterial metabolism of fecal urea.
The drop in luminal pH that accompanies
hydrolysis to short‐chain fatty acids in the colon
results in "trapping" of the ammonia by its
conversion to the polar ammonium ion.
Combined with the increases in colonic transit,
this therapy significantly lowers circulating
ammonia levels.

• Polyethylene Glycol–Electrolyte Solutions
• Long‐chain polyethylene glycols are poorly
absorbed, and PEG solutions are retained in the
lumen by virtue of their high osmotic nature.
When used in high volume, aqueous solutions of
PEGs with electrolytes produce an effective
catharsis and have replaced oral sodium
phosphates as the most widely used preparations
for colonic cleansing prior to radiological,
surgical, and endoscopic procedures.

• Usually 240 mL of this solution is taken every
10 minutes until 4 L is consumed or the rectal
effluent is clear. To avoid net transfer of ions
across the intestinal wall, these preparations
contain an isotonic mixture of sodium sulfate,
sodium bicarbonate, sodium chloride, and
potassium chloride. The osmotic activity of
the PEG molecules retains the added water
and the electrolyte concentration assures little
or no net ionic shifts.

• PEGs (without electrolytes) are also increasingly being
used in smaller doses (250‐500 mL daily) for the
treatment of constipation in difficult cases. A powder
form of polyethylene glycol 3350 is now available for
the short‐term treatment (2 weeks) of occasional
constipation, although the agent has been prescribed
safely for longer periods in clinical practice. The usual
dose is 17 g of powder per day in 8 ounces of water.
This preparation does not contain electrolytes, so
larger volumes may represent a risk for ionic shifts. As
with other laxatives, prolonged, frequent, or excessive
use may result in dependence or electrolyte imbalance.

Stool‐Wetting Agents and Emollients
• Docusate salts are anionic surfactants that
lower the surface tension of the stool to allow
mixing of aqueous and fatty substances,
softening the stool and permitting easier
defecation. However, these agents also
stimulate intestinal fluid and electrolyte
secretion. Glycerin suppository is most
commonly used stool softener without any
serious side effects.

• Mineral oil is a mixture of aliphatic hydrocarbons obtained
from petrolatum. The oil is indigestible and absorbed only
to a limited extent. When mineral oil is taken orally for 2‐3
days, it penetrates and softens the stool and may interfere
with resorption of water.
• The side effects of mineral oil preclude its regular use and
include interference with absorption of fat‐soluble
substances (such as vitamins), elicitation of foreign‐body
reactions in the intestinal mucosa and other tissues, and
leakage of oil past the anal sphincter. Rare complications
such as lipid pneumonitis due to aspiration also can occur,
so "heavy" mineral oil should not be taken at bedtime and
"light" (topical) mineral oil should never be administered
orally.

Stimulant (Irritant) Laxatives
• Stimulant laxatives have direct effects on
enterocytes, enteric neurons, and GI smooth
muscle. These agents probably induce a limited
low‐grade inflammation in the small and large
bowel to promote accumulation of water and
electrolytes and stimulate intestinal motility.
Proposed mechanisms include activation of
prostaglandin–cyclic AMP and NO–cyclic GMP
pathways, platelet‐activating factor production,
and inhibition of Na+, K+‐ATPase. Included in this
group are diphenylmethane derivatives,
anthraquinones, and ricinoleic acid.

• Diphenylmethane Derivatives
• Bisacodyl is the only diphenylmethane derivative
available.
• The usual oral daily dose of bisacodyl is 10‐15 mg
for adults and 5‐10 mg for children ages 6‐12
years old. The drug requires hydrolysis by
endogenous esterases in the bowel for activation,
and so the laxative effects after an oral dose
usually are not produced in <6 hours; taken at
bedtime, it will produce its effect the next
morning.

• Bisacodyl is mainly excreted in the stool; ~5%
is absorbed and excreted in the urine as a
glucuronide. Overdosage can lead to catharsis
and fluid and electrolyte deficits. The
diphenylmethanes can damage the mucosa
and initiate an inflammatory response in the
small bowel and colon.

• Anthraquinone Laxatives
• These derivatives of plants such as aloe, cascara,
and senna share a tricyclic anthracene nucleus
modified with hydroxyl, methyl, or carboxyl
groups to form monoanthrones, such as rhein
and frangula. Monoanthrones are irritating to the
oral mucosa; however, the process of aging or
drying converts them to more innocuous dimeric
(dianthrones) or glycoside forms. This process is
reversed by bacterial action in the colon to
generate the active forms.

• Senna is obtained from the dried leaflets on
pods of Cassia acutifolia or Cassia angustifolia
and contains the rhein dianthrone glycosides
sennoside A and B. Cascara sagrada is
obtained from the bark of the buckthorn tree
and contains the glycosides barbaloin and
chrysaloin. Barbaloin is also found in aloe. The
rhubarb plant also produces anthraquinone
compounds that have been used as laxatives.

• Anthraquinone laxatives can produce giant
migrating colonic contractions and induce
water and electrolyte secretion. They are
poorly absorbed in the small bowel, but
because they require activation in the colon,
the laxative effect is not noted until 6‐12
hours after ingestion. Active compounds are
absorbed to a variable degree from the colon
and excreted in the bile, saliva, milk, and
urine.

• The adverse consequences of long‐term use of
these agents have limited their use. A
melanotic pigmentation of the colonic mucosa
(melanosis coli) has been observed in patients
using anthraquinone laxatives for long periods
(at least 4‐9 months).

Castor Oil
• Castor oil is derived from the bean of the castor
plant, Ricinus communis. The castor bean is the
source of an extremely toxic protein, ricin, as well
as the oil (chiefly of the triglyceride of ricinoleic
acid). The triglyceride is hydrolyzed in the small
bowel by the action of lipases into glycerol and
the active agent, ricinoleic acid, which acts
primarily in the small intestine to stimulate
secretion of fluid and electrolytes and speed
intestinal transit.

• When taken on an empty stomach, as little as
4 mL of castor oil may produce a laxative
effect within 1‐3 hours; however, the usual
dose for a cathartic effect is 15‐60 mL for
adults. Because of its unpleasant taste and its
potential toxic effects on intestinal epithelium
and enteric neurons, castor oil is seldom
recommended now.

Prokinetic and Other Agents for
Constipation
• The term prokinetic generally is reserved for
agents that enhance GI transit via interaction
with specific receptors involved in the
regulation of motility.
• Newer agents, such as the potent 5‐HT4‐
receptor agonist prucalopride, may be useful
for the treatment of chronic constipation.

• Another potentially useful agent is misoprostol, a
synthetic prostaglandin analog primarily used for
protection against gastric ulcers resulting from
the use of NSAIDs
• Prostaglandins can stimulate colonic contractions,
particularly in the descending colon, and this may
account for the diarrhea that limits the
usefulness of misoprostol as a gastroprotectant.
However, this property may be utilized for
therapeutic gain in patients with intractable
constipation.

• Colchicine, a microtubule formation inhibitor
used for gout also has been shown to be
effective in constipation (mechanism
unknown), but its toxicity has limited
widespread use. A novel biological agent,
neurotrophin‐3 (NT‐3), recently was shown to
be effective in improving frequency and stool
consistency and decreasing straining, again by
an unknown mechanism of action.

• New development in the treatment of constipation is
the introduction of drugs that enhance fluid secretion
by acting locally on ion channels in the colonic
epithelium, to promote secretion.
• Lubiprostone is a prostanoid activator of Cl– channels.
The drug appears to bind to EP4 receptors linked to
activation of adenylyl cyclase, leading to enhanced
apical Cl– conductance. Lubiprostone was recently
introduced for treatment of chronic constipation in
adults and irritable bowel syndrome with constipation
in adult women.

Diarrhea
• Diarrhea (Greek and Latin: dia, through, and
rheein, to flow or run) does not require any
definition to people who suffer from "the too
rapid evacuation of too fluid stools."

• Scientists usually define diarrhea as excessive
fluid weight, with 200 g per day representing
the upper limit of normal stool water weight
for healthy adults. Because stool weight is
largely determined by stool water, most cases
of diarrhea result from disorders of intestinal
water and electrolyte transport.

• Diarrhea can be caused by
• an increased osmotic load within the intestine
(resulting in retention of water within the lumen);
excessive secretion of electrolytes and water into the
intestinal lumen
• exudation of protein and fluid from the mucosa; and
altered intestinal motility resulting in rapid transit (and
decreased fluid absorption).
• In most instances, multiple processes are affected
simultaneously, leading to a net increase in stool
volume and weight accompanied by increases in
fractional water content.

Bulk‐Forming and Hydroscopic Agents
• Hydrophilic and poorly fermentable colloids or
polymers such as carboxymethylcellulose and
calcium polycarbophil absorb water and increase
stool bulk (calcium polycarbophil absorbs 60
times its weight in water). They usually are used
for constipation but are sometimes useful in
acute episodic diarrhea and in mild chronic
diarrheas in patients suffering with IBS. They may
work as gels to modify stool texture and viscosity
and to produce a perception of decreased stool
fluidity. Some of these agents also may bind
bacterial toxins and bile salts.

• Clays such as kaolin (a hydrated aluminum
silicate) and other silicates such as attapulgite
(magnesium aluminum disilicate) bind water
avidly (attapulgite absorbs eight times its weight
in water) and also may bind enterotoxins.
However, binding is not selective and may involve
other drugs and nutrients; hence these agents
are best avoided within 2‐3 hours of taking other
medications. A mixture of kaolin and pectin (a
plant polysaccharide) is a popular over‐the‐
counter remedy and may provide useful
symptomatic relief of mild diarrhea.

Bile Acid Sequestrants
• Cholestyramine, colestipol, and colesevalam
effectively bind bile acids and some bacterial
toxins. Cholestyramine is useful in the
treatment of bile salt–induced diarrhea, as in
patients with resection of the distal ileum. In
these patients, there is partial interruption of
the normal enterohepatic circulation of bile
salts, resulting in excessive concentrations
reaching the colon and stimulating water and
electrolyte secretion.

• Patients with extensive ileal resection (usually
>100 cm) eventually develop net bile salt
depletion, which can produce steatorrhea
because of inadequate micellar formation
required for fat absorption. In such patients, the
use of cholestyramine aggravates the diarrhea.
The drug also has had an historic role in treating
mild antibiotic‐associated diarrhea and mild
colitis due to Clostridium difficile. However, its
use in infectious diarrheas generally is
discouraged because it may decrease clearance of
the pathogen from the bowel.

Bismuth
• Bismuth is thought to have anti‐secretory, anti‐
inflammatory, and antimicrobial effects. Nausea and
abdominal cramps also are relieved by bismuth.
Bismuth subsalicylate has been used extensively for the
prevention and treatment of traveler's diarrhea, but it
also is effective in other forms of episodic diarrhea and
in acute gastroenteritis. Today, the most common
antibacterial use of this agent is in the treatment of
Helicobacter pylori. For control of indigestion, nausea,
or diarrhea, the dose is repeated every 30‐60 minutes,
as needed, up to eight times a day.

Probiotics
• Probiotic preparations (Lactobacillus,
Bifidobacterium) containing a variety of
bacterial strains have shown some degree of
benefit in acute diarrheal conditions,
antibiotic‐associated diarrhea, and infectious
diarrhea, but most clinical studies have been
small and conclusions are therefore limited.
Because these agents are generally safe, their
use continues despite mainly anecdotal
evidence of efficacy.

Anti‐Motility and Anti‐Secretory
Agents
• Opioids
• Opioids continue to be widely used in the treatment of
diarrhea. They act by several different mechanisms,
mediated principally through either mu or delta opioid
receptors on enteric nerves, epithelial cells, and muscle.
• These mechanisms include effects on intestinal motility
(mu receptors), intestinal secretion ( delta receptors), or
absorption ( mu and delta receptors). Commonly used anti‐
diarrheals such as diphenoxylate, difenoxin, and
loperamide act principally via peripheral mu opioid
receptors and are preferred over opioids that penetrate the
CNS.

• Loperamide
• Loperamide (IMODIUM), a piperidine
butyramide derivative with mu receptor
activity, is an orally active anti‐diarrheal agent.
The drug is 40‐50 times more potent than
morphine as an anti‐diarrheal agent and
penetrates the CNS poorly.

• It increases small intestinal and mouth‐to‐cecum
transit times. Loperamide also increases anal
sphincter tone, an effect that may be of
therapeutic value in some patients who suffer
from anal incontinence. In addition, loperamide
has anti‐secretory activity against cholera toxin
and some forms of Escherichia coli toxin,
presumably by acting on Gi‐linked receptors and
countering the increase in cellular cyclic AMP
generated in response to the toxins.

• Loperamide has been shown to be effective
against traveler's diarrhea, used either alone or in
combination with antimicrobial agents
(trimethoprim, trimethoprim‐sulfamethoxazole,
or a fluoroquinolone). Loperamide also has been
used as adjunct treatment in almost all forms of
chronic diarrheal disease, with few adverse
effects. Loperamide lacks significant abuse
potential and is more effective in treating
diarrhea than diphenoxylate.

• Diphenoxylate
• Lomotil (Diphenoxylate 2.5 mg, atropine sulphate
25μg)
• Diphenoxylate and its active metabolite difenoxin
(diphenoxylic acid) are piperidine derivatives that
are related structurally to meperidine. As anti‐
diarrheal agents, diphenoxylate and difenoxin are
more potent than morphine.
• They also act on mu receptors and produce anti‐
diarrheal effect.

• Both compounds are extensively absorbed after oral
administration, with peak levels achieved within 1‐2
hours. Diphenoxylate is rapidly deesterified to
difenoxin, which is eliminated with a t1/2 of ~12 hours.
Both drugs can produce CNS effects when used in
higher doses (40‐60 mg per day) and thus have a
potential for abuse and/or addiction.
• They are available in preparations containing small
doses of atropine (considered subtherapeutic) to
discourage abuse and deliberate overdosage

Alpha 2 Adrenergic Receptor Agonists
• Alpha 2 Adrenergic receptor agonists such as
clonidine can interact with specific receptors on
enteric neurons and enterocytes, thereby
stimulating absorption and inhibiting secretion of
fluid and electrolytes and increasing intestinal
transit time. These agents may have a special role
in diabetics with chronic diarrhea, in whom
autonomic neuropathy can lead to loss of
noradrenergic innervation. Oral clonidine
(beginning at 0.1 mg twice a day) has been used
in these patients.

• Oral clonidine (beginning at 0.1 mg twice a
day) has been used in these patients.

Octreotide and Somatostatin
• Octreotide is an octapeptide analog of
somatostatin that is effective in inhibiting the
severe secretory diarrhea brought about by
hormone‐secreting tumors of the pancreas
and the GI tract. Its mechanism of action
appears to involve inhibition of hormone
secretion, including 5‐HT and various other GI
peptides (e.g., gastrin, vasoactive intestinal
polypeptide (VIP), insulin, secretin, etc.).

• Octreotide has been used in other forms of
secretory diarrhea such as chemotherapy‐
induced diarrhea, diarrhea associated with
human immunodeficiency virus (HIV), and
diabetes‐associated diarrhea.
• Octreotide has a t1/2 of 1‐2 hours and is
administered either subcutaneously or
intravenously as a bolus dose. Side effects of
octreotide depend on the duration of therapy.
Short‐term therapy leads to transient nausea,
bloating, or pain at sites of injection.

5‐HT3 Antagonists
• The 5‐HT3 receptor participates in several
important processes in the gut, including
sensitization of spinal sensory neurons, vagal
signaling of nausea, and peristaltic reflexes.
• Alosetron is a much more potent antagonist of
the 5‐HT3 receptor than ondansetron and causes
significant (although modest) improvements in
abdominal pain as well as stool frequency,
consistency, and urgency in these patients.

Bile Acid Therapy for Gallstones
• Ursodiol (ursodeoxycholic acid) is a naturally
occurring bile acid that makes up less than 5%
of the circulating bile salt pool in humans and
a much higher percentage in bears. After oral
administration, it is absorbed, conjugated in
the liver with glycine or taurine, and excreted
in the bile. Conjugated ursodiol undergoes
extensive enterohepatic recirculation. The
serum half‐life is approximately 100 hours.

• Ursodiol decreases the cholesterol content of
bile by reducing hepatic cholesterol secretion.
Ursodiol also appears to stabilize hepatocyte
canalicular membranes, possibly through a
reduction in the concentration of other
endogenous bile acids or through inhibition of
immune‐mediated hepatocyte destruction.

• Ursodiol is used for dissolution of small
cholesterol gallstones in patients with
symptomatic gallbladder disease who refuse
cholecystectomy or who are poor surgical
candidates. At a dosage of 10 mg/kg/d for 12–
24 months, dissolution occurs in up to 50% of
patients with small (< 5–10 mm) noncalcified
gallstones.

Antacids.pdf

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