Summary Anatomical changes in the gastrointestinal tract and subsequent weight loss may influence drug disposition and thus drug dosing following bariatric surgery. This review systematically examines the effects of bariatric surgery on drug pharmacokinetics, focusing especially on the mechanisms involved in restricting oral bioavailability. Studies with a longitudinal before‐after design investigating the pharmacokinetics of at least one drug were reviewed. The need for dose adjustment following bariatric surgery was examined, as well as the potential for extrapolation to other drugs subjected to coinciding pharmacokinetic mechanisms. A total of 22 original articles and 32 different drugs were assessed. The majority of available data is based on Roux‐en‐Y gastric bypass (RYGBP) (18 of 22 studies), and hence, the overall interpretation is more or less limited to RYGBP. In the case of the majority of studied drugs, an increased absorption rate was observed early after RYGBP. The effect on systemic exposure allows for a low degree of extrapolation, including between drugs subjected to the same major metabolic and transporter pathways. On the basis of current understanding, predicting the pharmacokinetic change for a specific drug following RYGBP is challenging. Close monitoring of each individual drug is therefore recommended in the early postsurgical phase. Future studies should focus on the long‐term effects of bariatric surgery on drug disposition, and they should also aim to disentangle the effects of the surgery itself and the subsequent weight loss.

Obesity is associated with comorbidities of which pharmacological treatment is needed. Physiological changes associated with obesity may influence the pharmacokinetics of drugs, but the effect of body weight on drug metabolism capacity remains uncertain. The aim of this study was to investigate ex vivo activities of hepatic drug metabolizing CYP enzymes in patients covering a wide range of body weight. Liver biopsies from 36 individuals with a body mass index (BMI) ranging from 18 to 63 kg/m 2 were obtained. Individual hepatic microsomes were prepared and activities of CYP3A, CYP2B6, CYP2C8, CYP2D6, CYP2C9, CYP2C19 and CYP1A2 were determined. The unbound intrinsic clearance (CL int,u) values for CYP3A correlated negatively with body weight (r ¼ À0.43, p < 0.01), waist circumference (r ¼ À0.47, p < 0.01), hip circumference (r ¼ À0.51, p < 0.01), fat percent (r ¼ À0.41, p < 0.05), fat mass (r ¼ À0.48, p < 0.01) and BMI (r ¼ À0.46, p < 0.01). Linear regression analysis showed that CL int,u values for CYP3A decreased with 5% with each 10% increase in body weight (r 2 ¼ 0.12, b ¼ À0.558, p < 0.05). There were no correlations between body weight measures and CL int,u values for the other CYP enzymes investigated. These results indicate reduced hepatic metabolizing capacity of CYP3A substrates in patients with increasing body weight.

It remains uncertain whether pharmacokinetic changes following Roux‐en‐Y gastric bypass (RYGB) can be attributed to surgery‐induced gastrointestinal alterations per se and/or the subsequent weight loss. The aim was to compare short‐ and long‐term effects of RYGB and calorie restriction on CYP3A‐activity, and cross‐sectionally compare CYP3A‐activity with normal weight to overweight controls using midazolam as probe drug. This three‐armed controlled trial included patients with severe obesity preparing for RYGB ( n = 41) or diet‐induced ( n = 41) weight‐loss, and controls ( n = 18). Both weight‐loss groups underwent a 3‐week low‐energy‐diet (<1200 kcal/day) followed by a 6‐week very‐low‐energy‐diet or RYGB (both <800 kcal/day). Patients were followed for 2 years, with four pharmacokinetic investigations using semisimultaneous oral and intravenous dosing to determine changes in midazolam absolute bioavailability and clearance, within and between groups. The RYGB and diet groups showed similar weight‐loss at week 9 (13 ± 2.4% vs. 11 ± 3.6%), but differed substantially after 2 years (−30 ± 7.0% vs. −3.1 ± 6.3%). At baseline, mean absolute bioavailability and clearance of midazolam were similar in the RYGB and diet groups, but higher compared with controls. On average, absolute bioavailability was unaltered at week 9, but decreased by 40 ± 7.5% in the RYGB group and 32 ± 6.1% in the diet group at year 2 compared with baseline, with no between‐group difference. No difference in clearance was observed over time, nor between groups. In conclusion, neither RYGB per se nor weight loss impacted absolute bioavailability or clearance of midazolam short term. Long term, absolute bioavailability was similarly decreased in both groups despite different weight loss, suggesting that the recovered CYP3A‐activity is not only dependent on weight‐loss through RYGB.

The liver and small intestine restrict oral bioavailability of drugs and constitute the main sites of pharmacokinetic drug-drug interactions. Hence, detailed data on hepatic and intestinal activities of drug metabolizing enzymes is important for modeling drug disposition and optimizing pharmacotherapy in different patient populations. The aim of this study was to determine the activities of seven cytochrome P450 (P450) enzymes in paired liver and small intestinal samples from patients with obesity. Biopsies were obtained from 20 patients who underwent Roux-en-Y gastric bypass surgery following a 3-week low-energy diet. Individual hepatic and intestinal microsomes were prepared and specific probe substrates in combined incubations were used for determination of CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A activities. The activities of CYP2C8, CYP2C9, CYP2D6, and CYP3A were quantified in both human liver microsomes (HLM) and human intestinal microsomes (HIM), while the activities of CYP1A2, CYP2B6, and CYP2C19 were only quantifiable in HLM. Considerable interindividual variability was present in both HLM (9-to 23-fold) and HIM (5-to 55-fold). The median metabolic HLM/HIM ratios varied from 1.5 for CYP3A to 252 for CYP2C8. The activities of CYP2C9 in paired HLM and HIM were positively correlated (r 5 0.74, P < 0.001), while no interorgan correlations were found for activities of CYP2C8, CYP2D6, and CYP3A (P > 0.05). Small intestinal CYP3A activities were higher in females compared with males (P < 0.05). Hepatic CYP2B6 activity correlated negatively with body mass index (r 5 20.72, P < 0.001). These data may be useful for further in vitro-in vivo predictions of drug disposition in patients with obesity. SIGNIFICANCE STATEMENT Hepatic and intestinal drug metabolism is the key determinant of oral drug bioavailability. In this study, paired liver and jejunum samples were obtained from 20 patients with obesity undergoing gastric bypass surgery following a 3-week low-energy diet. We determined the hepatic and small intestinal activities of clinically important P450 enzymes and provide detailed enzyme kinetic data relevant for predicting in vivo disposition of P450 substrates in this patient population.