Oral Ganciclovir Systemic Exposure Is Enhanced in HIV-Infected Patients With Diarrhea and Weight Loss:

Impact of gastrointestinal tract variability on oral drug absorption and pharmacokinetics: An UNGAP review

The absorption of oral drugs is frequently plagued by significant variability with potentially serious therapeutic consequences. The source of variability can be traced back to interindividual variability in physiology, differences in special populations (age- and disease-dependent), drug and formulation properties, or food-drug interactions. Clinical evidence for the impact of some of these factors on drug pharmacokinetic variability is mounting: e.g. gastric pH and emptying time, small intestinal fluid properties, differences in pediatrics and the elderly, and surgical changes in gastrointestinal anatomy. However, the link of colonic factors variability (transit time, fluid composition, microbiome), sex differences (male vs. female) and gut-related diseases (chronic constipation, anorexia and cachexia) to drug absorption variability has not been firmly established yet. At the same time, a way to decrease oral drug pharmacokinetic variability is provided by the pharmaceutical industry: clinical evidence suggests that formulation approaches employed during drug development can decrease the variability in oral exposure. This review outlines the main drivers of oral drug exposure variability and potential approaches to overcome them, while highlighting existing knowledge gaps and guiding future studies in this area.

Contributions of CYP3A4, P-glycoprotein, and serum protein binding to the intestinal first-pass extraction of saquinavir

Using CYP3A4-expressing Caco-2 cell monolayers, we assessed the roles of CYP3A4-mediated metabolism, P-glycoprotein (P-gp)-mediated efflux, and serum protein binding in determining the extent of the intestinal first-pass extraction (E(i)) of saquinavir. Saquinavir (5-40 microM) was added to the apical compartment of culture inserts. After 3 h, apical and basolateral media and cell scrapings were analyzed for saquinavir and a major CYP3A4-mediated metabolite (M7). The intracellular concentration of saquinavir was estimated from the degree of inhibition of CYP3A4 catalytic activity (midazolam 1'-hydroxylation). Compared with vehicle, the P-gp inhibitor LY335979 (zosuquidar trihydrochloride) (0.5 microM, apical) increased saquinavir cell content and M7 formation rate, but decreased the E(i) by approximately 50% due to a >90% increase in the amount of saquinavir recovered in the basolateral compartment. Compared with LY335779, physiological concentrations of basolateral serum proteins [human serum albumin and alpha1-acid glycoprotein (AAG)] increased saquinavir permeability by a similar degree but decreased the E(i) by approximately 50% due to a marked reduction in M7 formation. Increasing AAG concentration (1.0-2.5 g/l) had no additional effect on permeability or E(i). An estimate of the range of the E(i) of saquinavir (7-60%) was less than has been predicted based on in vitro data (>99%) but was consistent with a clinical study involving grapefruit juice. The incidental finding of greater M7 formation after basolateral compared with apical dosing could not be explained by differences in saquinavir cell content. We conclude that variable intestinal first-pass extraction of saquinavir in human immunodeficiency virus-infected patients could reflect variation in P-gp-mediated efflux and/or CYP3A4-catalyzed metabolism, but not in blood AAG levels.