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PharmacokineticsComputerized analysis of pharmacokinetic data13.2 Practice question1. The Practice calculations section of Chapter 6 presented the following data set for analysis by graphicalmeans:A patient has received a dose of 15mg of a drug by bolus i.v. injection. Blood samples were taken at various times afterwardsand analysed for drug concentration. The results are shown in Table 13.12.Time (h)Concentration (ng/mL)4 1528 12814 7420 4428 24Table 13.12 Drug concentrations at various times following 15mg i.v. bolus injectionFrom www.phrData.co.uk, click on ‘Pharmacokinetics’ and then ‘Curve fitting’. Notice that you can enter new datainto the sheet, but you should only change yellow cells – Do not change the contents of any white cells. Enterthe dose, blood sampling times and observed concentrations into the appropriate cells. The concentrations haveto be in units of mg/L so you will need to convert from ng/mL. Finally, enter whatever values you obtainedgraphically (Question 1, Chapter 6) for V and K as initial estimates (also yellow cells). Use Solver to optimizethe fit. What are the optimized values for V, K & Cl? How do these values compare to those you obtainedgraphically?Answers are available in the final section of the book.124Download free ebooks at bookboon.com
PharmacokineticsCreatinine clearance14 Creatinine clearanceThis chapter introduces the use of serum creatinine concentration as a marker of renal function. It is used to guide dosagecalculation for drugs that are extensively excreted via the kidneys. The material in this chapter is mainly relevant to clinicalpractice rather than drug development.14.1 Clearance of creatinine and various drugsWe are mainly concerned with those drugs that are simply filtered in the renal glomerulus after which there is no significantre-absorption from, or active secretion into, the urine. Because they are all handled in a similar manner, these drugs havevirtually identical renal clearances. Typical examples are the aminoglycoside antibiotics (including gentamicin) which arealmost exclusively eliminated in this manner. The renal excretion of digoxin also follows this pattern, although in its case,there is also significant hepatic elimination.Creatinine is a waste product produced in muscles. Like the drugs mentioned above, it is also excreted by simple filtrationin the kidneys and consequently it has the same renal clearance as the relevant drugs. If we can estimate creatinineclearance in a particular patient, then we can reasonably assume that these drugs will have the same renal clearance.Those clearances can then be used in dosage calculations. Digoxin dosing will be used (Section 14.2) to illustrate the useof serum creatinine as a marker of renal function.14.1.1 Estimation of creatinine clearanceThe precise estimation of creatinine clearance is a complex and time consuming process involving 24 hour urinecollection and analysis of blood samples and it would be unrealistic to attempt to carry out this procedure fora patient requiring a dosage calculation. However, Cockcroft and Gault (Cockcroft DW & Gault MH (1976)Nephron 16: 31–41) have provided a much simpler method to estimate creatinine clearance.Their method is based on the fact that serum creatinine concentrations will depend upon the rates of production andclearance of this waste product. The logical basis of the method is set out in Figure 14.1.• Body weight• Gender• AgeMusclemassCreatinineproductionrateCreatinineclearanceSerumcreatinineFigure 14.1 Interconnections between body weight, gender, age, creatinine clearance and serum creatinine concentration.125Download free ebooks at bookboon.com
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