Chapter 3

Biopharmaceutics and Pharmacokinetic Introduction

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Absorption and First-Pass Effect

redrawn from Barr, 1969

Figure 3.25.1 Slide showing extra absorption of salicylamide at higher doses. The green dots represent results after a 1 g dose and the red dots represent results after a 2 g dose. The original figure presented results after 0.3 and 0.5 g doses with resulting concentrations below those determined after the 1 g dose. Normally the amount of drug present in the body is no more than the enzymes can handle easily. Metabolism then proceeds in apparent first order fashion. However for some drugs the enzymes can not keep up. Illustrated here is one example, saturation of first pass metabolism allowing more or a higher fraction of drug to be absorbed.

• Barr, W.H. 1969 Factors involved in the assessment of systemic or biologic availability of drug products, Drug Inform. Bull., 3, 27-45

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