Pharmacokinetic factors considered in obese patients.

1. DOSING IN OBESE PATIENT
Dr. Ramesh Bhandari
Asst. Professor
Department of Pharmacy Practice
KLE College of Pharmacy, Belagavi

2. INTRODUCTION
• Obesity is a major problem.
• Obesity has been associated with increased mortality resulting from
increases in the incidence of hypertension, atherosclerosis,
coronary artery disease, diabetes, and other conditions compared to
non-obese patients.
• A patient is considered obese if actual body weight exceeds ideal or
desirable body weight by 20%, according to Metropolitan Life
Insurance Company data.
• Ideal or desirable body weights are based on average body weights
and heights for males and for females considering age.
• Athletes who have a greater body weight due to greater muscle mass
are not considered obese.

3. •Obesity often is defined by body mass index (BMI),
a value that normalizes body weight based on height.
•BMI is expressed as body weight (kg) divided by the
square of the person’s height (meters) or kg/m2.
•BMI can be calculated on the basis of following
formula:
BMI = {Weight (lb)/Height (in)2} x 703
BMI = {Weight (kg)/Height (cm)2} x 10,000

4. Weight Classification Based on BMI
Classification BMI (kg/m2)
Underweight <18.5
Normal body weight 18.5–24.9
Overweight 25–29.9
Obese 30–39.9
Extreme obesity >40

5. • BMI correlates strongly with total body fat in nonelderly
adults.
• Excess body fat increases the risk of death and major
comorbidities such as type 2 diabetes, hypertension,
dyslipidemia, cardiovascular disease, osteoarthritis of the knee,
sleep apnea, and some cancers.
• An obese patient (BMI > 30) has a greater accumulation of fat
tissue than is necessary for normal body functions.
• Adipose (fat) tissue has a smaller proportion of water
compared to muscle tissue.
• Thus, the obese patient has a smaller proportion of total body
water to total body weight compared to the patient of ideal
body weight, which could affect the apparent volume of
distribution of the drug.

6. • BMI is not a very accurate measure of adiposity in certain
individual patients, particularly in people with elevated lean body
mass, such as athletes, and in children.
• In addition to differences in total body water per kilogram body
weight in the obese patient, the greater proportion of body fat in these
patients could lead to distributional changes in the drug’s
pharmacokinetics due to partitioning of the drug between lipid and
aqueous environments.
• Drugs such as digoxin and gentamicin are very polar and tend to
distribute into water rather than into fat tissue.
• Although lipophilic drugs are associated with larger volumes of
distribution in obese patients compared to hydrophilic drugs, there
are exceptions and the effect of obesity on specific drugs must be
considered for accurate dosing strategy.

7. • Other pharmacokinetic parameters may be altered in the obese patient as a
result of physiologic alterations, such as fatty infiltration of the liver affecting
biotransformation and cardiovascular changes that may affect renal blood flow
and renal excretion.
• Dosing by actual body weight may result in overdosing of drugs such as
aminoglycosides (eg, gentamicin), which are very polar and are distributed in
extracellular fluids.
• Dosing of these drugs is based on ideal body weight.
• Lean body weight (LBW) has been estimated by several empirical equations
based on the patient’s height and actual (total) body weight.
LBW (Male) = 50 kg + 2.3 kg for each inch over 5 feet
LBW (Female) = 45.5 kg + 2.3 kg for each inch over 5 feet

8. Pharmacokinetic Changes in Obesity
1. Absorption:
 Bioavailability of medications in the obese population is scarce
and inconclusive.

9. Pharmacokinetic Changes in Obesity
2. Distribution:
 Measured as volume of distribution
 Influenced by size of the tissues, tissue perfusion, plasma protein binding
tissue permeability etc.
 Obese individuals have an increased total mass and adipose tissue mass
 Thus, volume of distribution for many drugs may be increased in the obese
population.
 However, lipophilicity of drugs plays a major role in the drug distribution.
 Lipophilic drugs showed larger volume of distribution whereas, hydrophilic
drugs showed less increased volume of distribution.
 Exception cyclosporine is highly lipophilic; Non-obese Vd– 295L and Obese
Vd – 229L.

10. Pharmacokinetic Changes in Obesity
2. Distribution:
 In addition, concentrations of plasma proteins – albumin, alpha -1
acid glycoprotein, and lipoproteins may be unchanged or
increased or decreased with obesity – results in altered
concentration of the unbound drugs.

11. Pharmacokinetic Changes in Obesity
3. Metabolism:
 Majority of the obese patients have fatty infiltration in the liver causing non
alcoholic fatty liver diseases with or without inflammation of the liver.
 Hence, Phase I and Phase II enzyme activity is affected in obese patient.
 Phase I Metabolism–
 CYP3A4 (carbamazepine and triazolam) activity reduced in obese patient.
 CYP2D6 – (Dexfenfluramine) Increased CYP2D6 activity in obese patient.
 CYP1A2 – (Caffeine and theophylline) higher clearance in obese patient
indicating increase in CYP1A2 activity.
 CYP2C9 – (Glimepiride and ibuprofen) significantly increased activity in obese
patient.
 CYP2C19 – (Diazepam) Clearance is higher in obese group.

12. Pharmacokinetic Changes in Obesity
3. Metabolism
 Phase II Metabolism –
 Uridine Diphosphate Glucuronosyl transferase (UGT) –
(acetaminophen, lorazepam) increased activity in the obese
patients.
 Liver blood flow in not fully determined due to non-alcoholic
fatty liver diseases.

13. Pharmacokinetic Changes in Obesity
4. Excretion
 Glomerular filtration rate is higher in the obese population which
reflects higher clearance of the drugs which are eliminated by this
route. (vancomycin, daptomycin and enoxaparin)
 Drugs like procainamide, ciprofloxacin and cisplatin showed
higher tubular secretion in the obese individuals.
 Tubular reabsorption of lithium is significantly lower in the
obese group. Hence renal clearance of the lithium was
significantly increased in obese patient.

14. Dosing consideration in the obese patients
Based on pharmacokinetic studies, principles of drug dosing for the
obese patients may be adopted to calculate loading and maintenance
dose.
a) Loading dose:
 Primarily based on Vd
 Weight used to calculate the loading dose depends on how the
drug is distributed into the lean mass.
 If the drugs is primarily distributed into the lean mass, IBW
should be used to calculate loading dose.
 In contrast if the drug is largely distributed into the fat tissues,
TBW should be used.

15. Dosing consideration in the obese patients
b) Maintenance dose:
 Depends on drug clearance
 Most common equation for estimating GFR is Cockcroft-Gault
equation.
 Clcr = [{(140-age in yrs) x weight in kg} / (72 x serum
creatinine)] x 0.85 (female)
 recommended to use TBW in underweight patients, IBW in
patients with normal weight and adjusted body weight for
overweight and obese patients.
 Adjusted body weight = IBW + 0.4 x (TBW-IBW) kg