BETA LACTAM & OTHER CELL WALL- & MEMBRANE ACTIVE ANTIBIOTICS MA. LENY ALDA G. JUSAYAN, MD DEPARTMENT OF PHARMACOLOGY
The penicillins constitute one of the most important groups of antibiotics. Although numerous other antimicrobial agents have been produced since the first penicillin became available, these still are widely used, major antibiotics, and new derivatives of the basic penicillin nucleus still are being produced. Many of these have unique advantages, such that members of this group of antibiotics are presently the drugs of choice for a large number of infectious diseases.
The penicillins were the first antibiotics discovered as natural products from the mold Penicillium.
1928 - Sir Alexander Fleming, professor of bacteriology at St. Mary's Hospital in London, was culturing Staphylococcus aureus. He noticed zones of inhibition where mold spores were growing. He named the mold Penicillium rubrum . It was determined that a secretion of the mold was effective against Gram-positive bacteria.
1928 - Alexander Fleming
Bread mold ( Penicillin notatum ) growing on petri dish
1939 - Florey, Chain, and Associates
Began work on isolating and synthizing large amounts of Penicillin.
1944 - Used in WWII to treat infections
Late 1940’s - available for general use in US
Penicillins as well as cephalosporins are called beta-lactam antibiotics and are characterized by three fundamental structural requirements:
the fused beta-lactam structure
a free carboxyl acid group
one or more substituted amino acid side chains
The lactam structure can also be viewed as the covalent bonding of pieces of two amino acids - cysteine and valine
Chemical Properties of Penicillins
The compound consists of 2 basic structures:
1. Thiazolidine Ring
2. Beta-Lactam Ring
- site of action of Beta- lactamase
- site of action of Amidase
- site of attachment of side chain, R, which determines many of the antibacterial and pharmacologic characteristics of a derivative (Spectrum and penicillin-resistance)
Chemical Properties of Penicillins
derivatives of benzylpenicillin, from which the methyl benzene radical is split off by amidase producing
6-aminopenicillanic acid, the parent compound of
all semisynthetic penicillins.
thiazolidine ring (A) connected to a b-lactam ring (B), to which is attached a side chain (R).
The penicillin nucleus itself is the chief structural requirement for biological activity;
metabolic transformation or chemical alteration of this portion of the molecule causes loss of all significant antibacterial activity
Cell Wall production
The cell walls of bacteria are essential for their normal growth and development.
Peptidoglycan is a heteropolymeric component of the cell wall that provides rigid mechanical stability by virtue of its highly cross-linked latticework structure
The peptidoglycan is composed of glycan chains, which are linear strands of two alternating amino sugars ( N-acetylglucosamine and N-acetylmuramic acid) that are cross-linked by peptide chains. (NAG-NAM).
In gram-positive microorganisms, the cell wall is 50 to 100 molecules thick, but it is only 1 or 2 molecules thick in gram-negative bacteria
The biosynthesis of the peptidoglycan involves about 30 bacterial enzymes and may be considered in three stages.
The first stage, precursor formation , takes place in the cytoplasm. The product, uridine diphosphate (UDP)-acetylmuramyl-pentapeptide , called a " Park nucleotide " accumulates in cells when subsequent synthetic stages are inhibited.
The last reaction in the synthesis of this compound is the addition of a dipeptide, D-alanyl-D-alanine . Synthesis of the dipeptide involves prior racemization of L-alanine and condensation catalyzed by D-alanyl-D-alanine synthetase.
The second stage, UDP-acetylmuramyl-pentapeptide and UDP-acetylglucosamine are linked (with the release of the uridine nucleotides) to form a long polymer.
The third and final stage involves the completion of the cross-link. This is accomplished by a transpeptidation reaction that occurs outside the cell membrane. The transpeptidase itself is membrane bound.)
Last step in peptidoglycan synthesis that is inhibited by the beta-lactam antibiotics .
Penicillin binds at the active site of the transpeptidase enzyme that cross-links the peptidoglycan strands
It mimicks D-alanyl-D-alanine residues that would normally bind to this site.
Penicillin irreversibly inhibits the enzyme transpeptidase by reacting with a serine residue in the transpeptidase. This reaction is irreversible and so the growth of the bacterial cell wall is inhibited.
Binding to PBPs results in:
Inhibition of transpeptidase: transpeptidase catalyzes the cross-linking of the pentaglycine bridge with the fourth residue (D-Ala) of the pentapeptide. The fifth residue (also D-Ala) is released during this reaction. Spheroblasts are formed.
Structural irregularities: binding to PBPs may result in abnormal elongation, abnormal shape, cell wall defects.
Beta Lactam Antibiotics & Other Cell Wall Synthesis Inhibitors
active against staphylococci and streptococci but inactive against enterococci & anaerobic bacteria, and gram-negative cocci & rods.
1. Benzylpenicillin – Penicillin G
2. Phenoxymethyl penicillin- Penicillin V
- greatest activity against gram-positive organisms, gram negative cocci, and
non-beta lactamase producing anaerobes and little activity against gram-negative rods.
- Susceptible to hydrolysis by beta-lactamases
B Extended Spectrum
c.1. Mezlocillin c.2. Azlocillin
c.3. Piperacillin c.4. Apalcillin
b.1.1 Indanyl Carbenicillin
b 1.2 disodium carbenicilli
b.1.3 Disodium Carbenicillin
retain the antibacterial spectrum of penicillin
improved activity against gram (-) organisms
destroyed by beta-lactamases
Mechanisms of Action:
inhibit the last step in the peptidoglycan synthesis of the cell wall
= inhibition of transpeptidase enzymes
= activation of penicillin binding proteins (PBPs)
= activation of autolysins (murein hydrolases)
Mechanisms of Drug Actions by Enzyme Inhibition:
All penicillin derivatives produce their bacteriocidal effects by inhibition of bacterial cell wall synthesis. Specifically, the cross linking of peptides on the mucosaccharide chains is prevented. If cell walls are improperly made cell walls allow water to flow into the cell causing it to burst.
Gastric juice at pH 2 rapidly destroys the antibiotic. The decrease in gastric acid production with aging accounts for better absorption of penicillin G from the gastrointestinal tract of older individuals.
Absorption is rapid, and maximal concentrations in blood are attained in 30 to 60 minutes. The peak value is approximately 0.5 unit/ml (0.3 mg/ml) after an oral dose of 400,000 units (about 250 mg) in an adult.
Oral Administration of Penicillin V . The sole virtue of penicillin V in comparison with penicillin G is that it is more stable in an acidic medium, and therefore is better absorbed from the gastrointestinal tract
Parenteral Administration of Penicillin G
After intramuscular injection, peak concentrations in plasma are reached within 15 to 30 minutes. This value declines rapidly, since the half-life of penicillin G is 30 minutes.
Repository preparations of penicillin G are employed. The two such compounds currently favored are penicillin G procaine ( maintained for as long as 4 to 5 days.) and penicillin G benzathine . (duration of antimicrobial activity in the plasma is about 26 days)
Such agents release penicillin G slowly from the area in which they are injected and produce relatively low but persistent concentrations of antibiotic in the blood.
Intrathecal administration is inadvisable particularly with benzylpenicillin as it can cause convulsions.
cannot penetrate the blood brain barrier
Probenecid and certain organic acids can inhibit transfer from CSF to blood stream
Bacterial meningitis: 1-5 mcg/ml = 18-24 M units/day
by the liver to penicillanic/penicillenic acid; penicillamine, penicilloid acid & other penicilloyl derivatives ( allergenic metabolites )
major antigenic determinant – penicilloyl polylysine (PPL)
2. Gastrointestinal disturbances after oral administration
3. Convulsions following rapid IV injection
4. Accidental injection into the sciatic nerve-severe pain and nerve dysfunction - persisting for weeks
5. Chronic use may cause:
- overgrowth of minor/atypical organisms following use of broad spectrum preparations
Procaine Penicillin G after accidental IV injection:
- pulmonary embolism
- acute psychotic reactions
Oxacillin and Nafcillin:
- granulocytopenia, bone marrow depression
Disodium Carbenicillin and high dose Penicillin G Na
Penicillin G Potassium
- hyperkalemia with high doses
Penicillin G Sodium
- Jarisch-herxheimer reaction
Carbenicillin and Ticarcillin
- bleeding diathesis
Methicillin - interstitial nephritis
- pseudomembranous colitis
Unitage of Penicillin
The international unit of penicillin is the specific penicillin activity contained in 0.6 mg of the crystalline sodium salt of penicillin G. One milligram of pure penicillin G sodium thus equals 1667 units; 1.0 mg of pure penicillin G potassium represents 1595 units. The dosage and the antibacterial potency of the semisynthetic penicillins are expressed in terms of weight.
The minimum inhibitory concentration(MIC) of any penicillin is usually given in ug/ml
Most penicillins ae dispensed as the sodium or potassium salt of the free acid.
effective doses : 4 - 24 million units/ day IV in 4 -6 div doses depending upon the organisms, the site and severity of infections
inhibitory for enterococci at 18 -24 mil units with an aminoglycoside
B. Procaine Penicillin G – IM
Peak in 1-3 hrs; T ½ - 12 hrs.
uncomplicated pneumococcal pneumonia or gonorrhea
C. Benzathine Penicillin G
mean duration of antimicrobial activity – 26 days
a single IM inj, 1.2 mil units for Beta-hemolytic streptococcal pharyngitis
IM once q 3-4 weeks – prophylaxis against reinfection with beta-hemolytic streptococci
2.4 Mil units IM once a week for 1-3 weeks- syphilis
D. Penicillin V
oral form in minor infections
relative poor bioavailability; dosing 4x a day
Gram (+) aerobic activities similar to Penicillin G
5-10x less active against gram (-) microbes, esp. Neisseria and certain anaerobes
better absorbed from the GIT
E. Ampicillin and Amoxicillin
same spectrum of activity
oral for UTI, sinusitis, otitis and lower resp inf
most active of the oral beta lactams vs pen resistant pneumococci s
Ampicillin but not amoxicillin is effective for shigellosis
E. Ampicillin and Amoxicillin
not used to treat uncomplicated salmonella gastroenteritis
somewhat less active than Pen G vs gram (+) cocci
enterococcal grp. D and viridans grp of streptococci
H. influenza. And E. coli
Adults - 1 - 4 gm/day
Severe infections :
Adult - 6 - 12 gm/day
F. ESTERS OF AMPICILLIN
No inherent antimicrobial activity as esters, but pharmacologically active following hydrolysis to ampicillin
50% higher blood concentration than Ampicillin and Amoxicillin
G. CARBOXYPENICILLINS and UREIDOPENICILLINS
Gram (-) aerobes
Bacteroides fragilis; but in higher amount/dose
Carboxypenicillins – certain indole (-) Proteus
Ureidopenicillins - Klebsiella
H. PENICILLINASE RESISTANT
indication is infection by beta lactamase producing staphylococci,
streptococci and pneumococci also susceptible
Isoxazolyl penicillins .25 - 0.5 g orally every 4-6 hrs
15 - 25mg/kg/d for children – mild localized staphylococcal infections
H. PENICILLINASE RESISTANT
relatively acid stable and well absorbed, food interferes with absorption
Oxacillin or Nafcillin, 8-12 g/d given by intermittent IV infusion of 1-2 g every 4-6 hours
serious systemic staphylococcal Infxns
Methicillin no longer used bec. of its nephrotoxicity
Penicillins bind to and inactivate aminoglycosides. This is a form of chemical antagonism. If an aminoglycoside and a penicillin are combined. they MUST NOT be administered simultaneously through the same I.V. line or through the same syringe. They will crystallize and precipitate in the line or in the vessels!
When an aminoglycoside and a penicillin are administered, the infusions should be staggered by about 1 to 2 hours.
Classification of the Penicillins and Summary of Their Pharmacological Properties
. Penicillin G and its close congener penicillin V are highly active against sensitive strains of gram-positive cocci, but they are readily hydrolyzed by penicillinase. Thus, they are ineffective against most strains of Staphylococcus aureus.
2. The penicillinase-resistant penicillins (methicillin, nafcillin, oxacillin, cloxacillin, and dicloxacillin) have less potent antimicrobial activity against microorganisms that are sensitive to penicillin G, but they are effective against penicillinase-producing Staph. aureus.
3. Ampicillin, amoxicillin, bacampicillin, and others comprise a group of penicillins whose antimicrobial activity is extended to include such gram-negative microorganisms as Haemophilus influenzae, E. coli, and Proteus mirabilis. Unfortunately, these drugs and the others listed below are hydrolyzed readily by broad-spectrum b-lactamases that are found with increasing frequency in clinical isolates of these gram-negative bacteria.
- contains an R2 that makes the compound stable in dilute acid and highly penicillinase resistant
- MW 400-450
- Soluble to water and relatively stable to ph and temperature changes
- Mechanisms of Action and Resistance similar to penicillins
GENERAL PROPERTIES OF CEPHALOSPORINS
- generally broader spectrum than Penicillins
- generally more effective than Penicillins against B-lactamase-producing microbes ( except enterococci, Methicillin-resistant Staph. Aureus and Staph epidermidis )
Good actvity vs gm (+) & modest vs gm (-) microbes
Penetration to the CSF is inadequate
Oral drugs used for the treatment of UTI, for minor staphylococcal lesions, or for minor polymicrobial infections such as cellulitis or soft tissue abscess.
not used in serious systemic infections
Cefazolin penetrates well into most tissues- the drug of choice for surgical prophylaxis
Cefazolin – only first generation parenteral cephalosphorin
Better activity vs anaerobes
first gen drugs actv but with extended gm (-) coverage
Cefamandole, cefuroxime, cefonicid, ceforanide & cefaclor – H. influenzae
Cefoxitin, cefmetazole and cefotetan – B. fragilis and some serratia strains
Oral dosage for adults is 10-15 mg/kg/d in two to four divided doses;
Children 20-40 mg/kg/day up to a maximum of 1 g /day.
Oral second generations are active vs beta-lactamase-producing H. influenzae or Branhamela catarrhalis and have been primarily used to treat sinusitis, otitis, or lower respiratory tract infections
Cefoxitin, Cefotetan or Cefmetazole – mixed anaerobic infections as peritonitis or diverticulitis
Cefuroxime – crosses the BBB & for community-acquired pneumonia
less active than the first generation vs gm (+) cocci but most active against gram (-) including B-lactamase- producing strains
Cefoperazone, Ceftazidime – more active against Pseudomonas
Cefoperazone, Cefotaxime – active against anaerobes
Ceftizoxime, Moxolactam – B. fragilis
Cross BBB except Cefoperazone, Cefixime, Ceftibuten and Cefpodoxime proxetil
Excretion – Cefoperazone and Ceftriaxone: bile;
Probenecid does not affect renal excretion
Ceftriaxone, 125 mg inj., Cefixime, single 400 mg - N. gonorrhea
Cefoperazone 25- 100 mg/kg/d injected q 8-12 hrs.
Cefixime 200 mg orally twice a day or 400 mg OD
Cefpodoxime proxetil & Ceftibuten – 200 mg 2x/day
Meningitis caused by pneumococci, meningococci, H. influenza & susceptible
enteric gm (-) rods but not by L. monocytogenes; used in combn
with aminoglycosides for Rx of meningitis caused by P. aeruginosa
- More resistant to hydrolysis by chromosomal beta lactamases (eg. Those produced by enterobacter)
Good activity against P. aeruginosa, enterobacteriaceae, Staph aureus, S. pneumoniae
Highly active against Haemophilus & Neisseria
Penetrates well into CSF
Cleared by kidneys
T ½ - 2 hrs.
Good activity against most penicillin resistant strains of streptococci
ADVERSE EFFECTS of Cephalosporins
> Renal toxicity – interstitial nephritis and even tubular necrosis
> Cephalosporin that contains a methyl thiotetrazole group ( Cefamandole, Moxolactam, Cefmetazole, Cefoteta, Cefoperazone) – cause disulfiram like reactions, hypoprothrombinemia and bleeding disorders – Antidote: Vit. K 10 mg 2x/ week
ADVERSE EFFECTS of Cephalosporins
> Moxolactam – interferes with platelet function, severe bleeding
Monocyclin beta lactam ring
Resistant to beta-lactamases and active against gram negative rods including Pseudomonas and Serratia
No activity against gram positive bacteria or anaerobes
Aztreonam : This drug is a monocyclic beta-lactam (a monobactam).
Mechanism of action :
Aztreonam interacts with penicillin binding proteins and induces the formation of long filamentous bacteria.
Antimicrobial spectrum :
The antimicrobial spectrum of aztreonam differs from that of other beta-lactams.
It more closely resembles the spectrum of the aminoglycosides. Gram positive and anaerobic bacteria are resistant. Susceptible organisms include: ( It has an unusual spectrum being active only against Gram-negative aerobic rods) Enterobacteriaceae, Pseudomonas, Hemophillus and Neisseria. Aztreonam is resistant to the beta-lactamase produced by gram negative organisms
Side effects: Generally, the drug is well tolerated. Patients who are allergic to penicillins do not exhibit cross-reactions with aztreonam.
Aztreonam – resembles aminoglycosides in spectrum of activity
Given 1-2 g IV every 8 hrs. T1/2 – 1-2 hrs.
BETA LACTAMASE INHIBITORS
Resemble B-lactam molecules but themselves have very weak antibacterial action
Bind to Beta-lactamase, inactivate them and prevent the destruction
Synergistic with other beta-lactams
BETA LACTAMASE INHIBITORS
Available only in fixed combinations with specific penicillins
A. Clavulanic Acid – combined with Amoxicillin (Augmentin) - combined with Ticarcillin (Timentin)
B. Sulbactam Pivoxil and Ampicillin (Unasyn)
C. Tazobactam and Piperacillin (Tazocin)
Carbepenems are a new class of drugs which are structurallv similar to the penicillins.
These drugs were developed to deal with beta‑lactamase producing Gram-negative organisms, which were resistant to broad spectrum and extended spectrum penicillins.
Carbapenems are derived from Streptomyces species and one example is the semisynthetic imipenem which acts in the same way as the other beta-lactams.
The most extensively studied drug is imipenem.
Mechanism of action: Imipenem, like other b-lactams, binds to penicillin binding proteins. Hence it disrupts cell wall synethesis and is bactericidal.
Antimicrobial spectrum: Imipenem differs from the penicillins in its antimicrobial spectrum. It is a broad-spectrum antibiotic with excellent activity against a variety of gram positive and gram negative organism (both aerobic and anaerobic).
It is resistant to most forms of b-lactamase, including that produced by staphylococcus. However, methicillin-resistant staphylococcus is usually resistant to imipenem .
Metabolism: Imipenem is rapidly hydrolyzed by the enzyme, dihydropeptidase, which is found in the brush border of the proximal renal tubule . It is always administered with cilastatin, an inhibitor of dipeptidase.
Side efects: Individuals who are allergic to the penicillins may demonstrate cross-reactivity with imipenem.
Imipemem may produce nausea and vomiting.
Seizures have been reported with high doses, particularly in patients with renal failure.
It is similar to imipenem.
It is not degraded by dehydropeptidase, thus no cilastatin is needed.
Excessive levels in kidney failure can cause seizures with imipenem but not with meropenem.
Imipenem – gram (-) rods, gram- positive organisms and anaerobes
> inactivated by dehydropeptidases in renal tubules resulting in low urinary concentrations.
combined with Cilastatin (dehydropeptidase inhibitor) to reduce inactivation
> penetrates body tissues and fluids well including the CSF
0.25 – 0.5 g IV q 6-8 hrs.
> adverse effects include nausea, vomiting, diarrhea, reactions to infusion site, excessive in renal failure – seizures
Meropenem – slightly greater activity against gram (-) aerobes; does not require an inhibitor
> penetrates CSF; 1 g q 8 hrs.
- gram (+) staph
MOA – binds firmly to D-Ala-D-Ala terminus of nascent peptidoglycan pentapeptide
Resistance – modification of the D-Ala-D-Ala binding site of the peptidoglycan building block in which the terminal D-Ala is replaced by D-lactate resulting to loss of a critical H bond that facilitates high affinity binding of vancomycin to its target and loss of activity
Poorly absorbed from the intestinal tract but oral for the Rx of antibiotic asst enterocolitis caused by C. difficile
99% excreted by glomerular filtration
t½: 6-10 days - not removed by dialysis
Rx for parenteral vancomycin: sepsis or endocarditis caused by meth-resistant staph
plus Cefotaxime, Ceftriaxone or Rifampicin for Rx of meningitis of pen resistant strain of pneumococcus.
Dose: 30 mg/kg/day in 2 -3 div doses
Causes phlebitis, chills and fever, ototoxicity, nephrotoxicity, red man or red neck syndrome
inhibits cytoplasmic enzyme enol pyruvate transferase by covalently binding to the cysteine residue of the active site and blocking the phosphoenolpyruvate to UDP-N- acetylglucosamine – 1st step in the formation of UDP-N-acetylmuramic acid, the precursor of acetylmuramic acid
- Transported into the cell by glycerophosphate or glucose 6 phosphate transport systems
Resistance is due to inadequate transport of drug into the cell
Active against both Gram (+) and Gram (-)
In vitro synergism with beta-lactams, animoglycosides or fluoroquinolones
Available orally (2-4 g, single dose in uncomplicated UTI) and parenterally
Excreted through the kidneys
Safe in pregnancy
active against Gram (+) organisms
inhibits cell wall formation by interfering with dephosphorylation in cycling of the lipid carrier that transfers peptidoglycan subunits to the growing cell wall
poorly absorbed; limited to topical use
500 units/g ointment + polymyxin B or Neomycin
widely distributed into tissues
excreted through the urine
0.5 – 1 g / d in 2-3 divided doses
causes dose related CNS toxicity, headaches, tremors, acute psychosis, convulsions
PARENT COMPOUND OF CEPHALOSPHORIN
PARENT COMPOUND OF PENICILLIN
3-6: composition of the beta lactam structure of the beta lactam antibiotics
7. ENZYME PRODUCED BY BACTERIA THAT HYDROLYSIS PENICILLIN
8. ANTIBIOTIC OF CHOICE AGAINST STAPH INFECTION
9. 2 ND GENERATION CEPHALOSPHORIN THAT CAN ENTER THE BBB