Problem 1
PROGRAMMED PROBLEM SET ON PHARMACOKINETICS
Carol T. Walsh, Ph.D.
Professor of Pharmacology
Boston University School of Medicine
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This programmed problem set is designed to help you learn and apply the concepts and vocabulary in Pharmacokinetics that have been introduced to you in lectures and text. The primary terms that you should master from this exercise are Half-life (t1/2), Volume of Distribution (Vd), Renal Clearance (ClR), Total Clearance (ClT), and Bioavailability (F).
Their definitions and methods for calculating these parameters are presented in the Pharmacology Glossary posted online and the lecture syllabus. You should complete this program before the first Pharmacology Discussion Session. Bring to that class any questions that arise during your use of this program. The first Discussion Session is designed to reinforce and expand upon the groundwork in pharmacokinetics presented in this problem set.
In order to help organize your interpretation of the data, a summary table is available for your use to enter in pharmacokinetic parameters as you determine them. Click here now to bring up the summary table page.
PHARMACOKINETICS OF CHLORAMPHENICOL
Chloramphenicol is a clinically useful antibiotic with the following chemical structure:
The compound is uncharged over the pH range 2-9 and is poorly soluble in water.
The concentration of chloramphenicol added to biological fluids may be determined in several ways:
- Bioassay: determination of the capacity of a sample to inhibit growth of microorganisms in vitro as compared to the capacity of a series of standards containing known amounts of chloramphenicol. Biotransformation products of chloramphenicol, produced in man and experimental animals, are inactive against microorganisms, so a bioassay of a sample containing both chloramphenicol and its metabolites measures unchanged chloramphenicol only. (Why would such an assay for chloramphenicol possibly be inappropriate in a patient receiving multiple antibiotic agents?)
- Chemical assay for nitro groups: NO2 groups are not present in mammalian fluids, so a chemical assay for this substituent can be used to quantitate chloramphenicol. But there is a problem with the accuracy, specifically the validity, of this assay because it detects all derivatives of chloramphenicol formed in vivo in which the NO2 remains but other parts of the molecule are modified. (Where might this molecule be modified? HINT: Might the aliphatic hydroxyl group be a site for glucuronide conjugation?)
- High-performance liquid chromatography (HPLC) or radioenzymatic assays: the advantage of these procedures is their capacity to assay specifically the concentration of the chloramphenicol molecule by chemical methods, and to distinguish chloramphenicol from its major metabolite (chloramphenicol glucuronide) and from inactive esteratic prodrug forms (chloramphenicol succinate or palmitate).
For this programmed problem set, you will be analyzing preclinical data collected during the initial development of this drug. The biological disposition of chloramphenicol was studied in dogs, as is currently required by F.D.A. regulations for all new drugs. Experiments from two normal male dogs are presented below.
One animal, weighing 16.5 kg, received a single dose of 50 mg/kg (825 mg total) intravenously (i.v.); the second, weighing 18.0 kg, received 50 mg/kg (900 mg total) of chloramphenicol orally (p.o.). For both animals, serum and urine samples were collected at various times after drug administration and analyzed by bioassay for their content of microbiologically active material (unchanged chloramphenicol). (These data are taken from the studies published in the Journal of Pharmacology and Experimental Therapeutics by the investigators at Parke-Davis who were responsible for the preclinical development of this drug prior to its marketing.)
| Time after administration of chloramphenicol (hr): | 0.5 | 1.0 | 1.5 | 2.0 | 3.0 | 4.0 | 6.0 | 8.0 | |
| Serum concentration of chloramphenicol (µg/ml): | |||||||||
| After i.v. administration | 32.2 | 24.2 | 17.0 | 12.5 | 6.6 | 3.5 | 0 | –* | |
| After p.o. administration | 0 | 1.0 | – | 10.0 | – | 15.0 | 8.7 | 4.5 | |
* indicates not determined
Complete urine collections were made during the period following administration of chloramphenicol. The concentration of chloramphenicol in each urine sample was determined and multiplied by urine volume to determine the amount of drug excreted in the urine in each interval (see below).
| Time after administration of chloramphenicol (hr): | 0-2 | 2-4 | 4-6 | 6-8 | 8-24 | 0-24 total amt (% of dose) |
|
| Amount of chloramphenicol in urine (mg): | |||||||
| After i.v. administration | 55 | 17 | 5 | – | – | 77 (9.3%) |
|
| After p.o. administration | 6 | 24 | 12 | 9 | 16 | 67 (7.4%) |
|
The following program will guide you through an evaluation of the data to conclusions about the pharmacokinetics of chloramphenicol. Your first step for each question should be to inspect the data and estimate the parameter required to arrive at the answer. These are the skills you will most often use in interpreting clinical data or the clinical literature. (You may better understand these data if you graph them and complete a regression analysis of ln Cp vs. time. You may find the Stat Tools spreadsheet used in the BUSM 1 Evidence-Based Medicine curriculum helpful for this purpose. A table of the Cp vs t data for copying into Excel is provided for your convenience.) If you find that you have difficulty working with these data, please consult a faculty member or tutor before the first discussion session.
NOW GO TO ITEM I
I. The elimination of chloramphenicol from the serum after it is given i.v. conforms to the laws of:
a. Zero-order kinetics
b. First-order kinetics
c. Mixed-order kinetics (a combination of zero-order and first-order kinetics)
II. The elimination half-life of chloramphenicol is:
a. About 30 minutes
b. About 1 hour
c. Neither of the above
III. In the 16.5 kg dog the apparent volume of distribution (Vd) of chloramphenicol is:
a. Less than 1 liter
b. About 4 liters
c. About 18 liters
d. About 50 liters
e. None of the above
IV. In the dog, as in a human adult male of average body weight and composition, the volumes of plasma, extracellular and total body water average 4%, 17%, and 58% of body weight, or 0.04, 0.17, and 0.58 l/kg respectively. Therefore, the apparent volume of distribution of chloramphenicol in the dog corresponds approximately to which of the following body fluid compartments:
a. Plasma volume
b. Intracellular fluid volume
c. Extracellular fluid volume
d. Total body water
e. Greater than total body water
V. Analysis of the serum concentration data for chloramphenicol after its i.v. and oral administration suggests that:
a. The elimination half-life of chloramphenicol depends on the route of administration
b. The absorption of chloramphenicol into the systemic circulation is incomplete, since peak concentration is lower after p.o. than i.v. administration
c. The absorption of chloramphenicol is complete, since the area under the concentration vs. time curve after the p.o. route (83 mg hr/ml) is about equivalent to that after i.v. administration (70 mg hr/ml) *
d. The absorption of chloramphenicol after oral administration exhibited a lag phase and was not rapid in onset
(*Note that you need not verify these area values; they have been correctly estimated.)
VI. The excretion of chloramphenicol in the urine following i.v. as compared to oral administration suggests that:
a. Regardless of the route of administration, chloramphenicol is primarily eliminated from the body by a mechanism other than renal excretion
b. Chloramphenicol is incompletely absorbed from the intestine
c. Chloramphenicol is incompletely absorbed and/or is eliminated by first-pass biotransformation in the liver
d. Urine collected 6-24 hr. after i.v. administration of chloramphenicol would be expected to contain substantial amounts of unchanged drug
VII. Some understanding of the mechanisms by which a drug is cleared from the plasma into the urine can be acquired by calculating of the renal clearance. The renal clearance (ClR) indicates the volume of the plasma that must have been cleared of the drug in order to achieve the output observed in the urine. Recall your calculations of renal clearance of nutrients such as glucose in physiology.
| ClR (ml/min) | = | rate of renal excretion (amount/min) |
| plasma concentration (amount/ml) |
(Note that usually the concentrations of a drug in serum and plasma are about the same.)
| ClR | 70-kg human | 16.5-kg dog | Estimate for: | |
| Inulin | 120 ml/min | 50 ml/min | GFR | |
| Para-aminohippuric acid | 650 ml/min | 200 ml/min | RPF | |
The renal clearance of chloramphenicol in the dog after i.v. administration is:
a. 0 to 9 ml / min
b. 10 to 35 ml / min
c. 36 to 54 ml / min
d. 55 to 200 ml / min
e. Greater than 200 ml / min
f. Likely to be significantly different than after oral administration
VIII. The value of renal clearance of chloramphenicol in the dog indicated that:
a. Chloramphenicol is filtered by the glomeruli
b. Chloramphenicol is incompletely filtered by the glomeruli and/or partially reabsorbed
c. Chloramphenicol is filtered, partially reabsorbed, and secreted
IX. The rate of renal excretion of chloramphenicol and its renal clearance most likely:
a. Increase if the pH of the urine is decreased, as occurs with basic drugs such as amphetamine (pKa=10)
b. Are unrelated to the pH of the urine
c. Increase if the pH of the urine increases, as occurs with acidic drugs such as salicyclic acid (pKa=3)
X. The total clearance (ClT) of a drug indicates the clearance from the body by all routes and mechanisms. This value can be estimated from:
ClTOT = (0.693/t1/2)Vd
(Suggestion: Use half-life in min and Vd in ml so your estimate of ClTOT is in ml/min. Do you get a value for chloramphenicol in the dog of about 195 ml/min?).
Or if kel has already been calculated, it can be computed from:
ClTOT = kel Vd, where kel is the slope of the ln Cp vs. t plot after i.v. administration.
Comparison of the total clearance of chloramphenicol (195ml/min) to its renal clearance (See comment to Item VIIIb) indicates that:
a. Chloramphenicol is cleared only by the kidney, and not by the liver or other organs
b. Chloramphenicol is cleared by the liver, as well as by the kidney
c. Chloramphenicol is cleared from the body partly by biotransformation
d. Nonrenal mechanisms account for about 90% of the clearance of chloramphenicol
XI. In this experiment the chemical assay for nitro groups was also used to determine serum and urine content of nitro-containing compounds (both chloramphenicol and its metabolites). For a given sample, total nitro content minus chloramphenicol content indicates metabolite content. The data for chloramphenicol glucuronide, the major metabolite, indicate a renal clearance of 96.3 ml/min and a cumulative urinary excretion of 60.5% of the chloramphenicol dose after i.v. administration in the 16.5 kg dog. These data suggest that:
a. Chloramphenicol glucuronide is eliminated in part by renal tubular secretion, but a considerable portion of the dose is eliminated by another mechanism, possibly in the bile
b. The bacteriostatic effect of chloramphenicol in the urine is primarily caused by its metabolites
c. Renal impairment in a patient is more likely to increase the serum concentrations of chloramphenicol than of its metabolite
XII. In another experiment, the i.v. injection of 25 mg/kg of chloramphenicol to a 16.5 kg dog was found to produce bacteriostatic serum concentrations. In comparison to a dose of 50 mg/kg, this dose would probably result in:
a. A shorter elimination half-life
b. A smaller renal clearance
c. A smaller volume of distribution
d. A halving of the duration of bacteriostatic effect in serum
e. A 1-hr reduction of the duration of bacteriostatic effect in the serum
