Programmed Problem Set on Pharmacokinetics

Carol T. Walsh, Ph.D.
Professor of Pharmacology
Boston University School of Medicine

Questions or comments should be mailed to Carol Walsh.

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. Fill in the table as you work through the problem set, and then bring it with you to the discussion session on pharmacokinetics.

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 pharmacokinetic 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. Use this Cp vs Time spreadsheet and enter the plasma concentration (Cp) data from above, before moving on to Item I. 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:


II. The elimination half-life of chloramphenicol is:


III. In the 16.5 kg dog the apparent volume of distribution (Vd) of chloramphenicol is:

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:

V. Analysis of the serum concentration data for chloramphenicol after its i.v. and oral administration suggests that:

(*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:


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:


VIII. The value of renal clearance of chloramphenicol in the dog indicated that:

IX. The rate of renal excretion of chloramphenicol and its renal clearance most likely:


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:

ClT = (0.693/t1/2)Vd

(Suggestion: Use half-life in min and Vd in ml so your estimate of ClT 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:

ClT = 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:

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:


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: