Mechanistic Oral Absorption Modeling and Simulation for Formulation Development and Bioequivalence Evaluation: Report of an FDA Public Workshop

On May 19, 2016, the US Food and Drug Administration (FDA) hosted a public workshop, entitled “Mechanistic Oral Absorption Modeling and Simulation for Formulation Development and Bioequivalence Evaluation.”1 The topic of mechanistic oral absorption modeling, which is one of the major applications of physiologically based pharmacokinetic (PBPK) modeling and simulation, focuses on predicting oral absorption by mechanistically integrating gastrointestinal transit, dissolution, and permeation processes, incorporating systems, active pharmaceutical ingredient (API), and the drug product information, into a systemic mathematical whole‐body framework.2

Altered morphine glucuronide and bile acid disposition in patients with nonalcoholic steatohepatitis

The functional impact of altered drug transport protein expression on the systemic pharmacokinetics of morphine, hepatically derived morphine glucuronide (morphine-3- and morphine-6-glucuronide), and fasting bile acids was evaluated in patients with biopsy-confirmed nonalcoholic steatohepatitis (NASH) compared to healthy subjects. The maximum concentration (Cmax ) and area under the concentration-time curve (AUC0-last ) of morphine glucuronide in serum were increased in NASH patients (343 vs. 225 nM and 58.8 vs. 37.2 µM*min, respectively; P ≤ 0.005); morphine pharmacokinetics did not differ between groups. Linear regression analyses detected an association of NASH severity with increased morphine glucuronide Cmax and AUC0-last (P < 0.001). Fasting serum glycocholate, taurocholate, and total bile acid concentrations were associated with NASH severity (P < 0.006). Increased hepatic basolateral efflux of morphine glucuronide and bile acids is consistent with altered hepatic transport protein expression in patients with NASH and may partially explain differences in efficacy and/or toxicity of some highly transported anionic drugs/metabolites in this patient population.

Pharmacokinetics (PK) of the highly lipophilic and blood stable camptothecin AR-67 (7-t-butyldimethylsilyl-10- hydroxycamptothecin) in adult patients with solid malignancies

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Background: Camptothecin analogs possess a labile lactone ring, which readily undergoes a pH dependent, albeit reversible, hydrolysis in plasma to yield a carboxylate moiety. The latter is considered inactive due to its electronegative charge that impedes transport into cells. Furthermore, the carboxylate is cleared rapidly and causes toxicity in eliminating organs due to lactonation. AR-67 is a highly lipophilic 3rd generation analog with superior stability of its lactone form in preclinical models. This report describes the PK of AR-67 in patients with refractory solid tumors enrolled in a phase I study. Methods: AR-67 was infused over 1 hr for 5 days every 21-days. PK was performed on the 1st and 4th day of cycle 1. Blood, plasma, and urine were collected (0–24 hrs) from 26 patients (see 09-AB-30336-ASCOAM) treated at 9 dose levels: 1.2–12.4 (mg/m2/day). AR-67 carboxylate and lactone were assayed with a validated chromatography method. Results: AR-67 was detectable at all dose levels. Blood concentrations mirrored those in plasma and were superimposable when adjusted by the hematocrit. AR-67 concentration peaked at the end of the 1-hr infusion and declined biexponentially with a terminal t1/2 of 1.4 hr (plasma lactone). A linear relationship was observed between dose and AUC. The lactone clearance on Day 1 was 16.6 (±5.5) vs. 19.6 (±6.3) L/hr/m2 on Day 5. The carboxylate clearance was ∼ 6-fold higher. Lactone was the major form in all samples and its area under the time vs. concentration curve (AUC) was 85.5% (range 74.0%-94.1%) of the total AUC. Urine (0–24 hr) contained 2.5% (0.3%-6.7%) of the dose on Day 1 vs. 2.7% (0.9–11.1%) on Day 4. Extensive metabolite peaks were not observed in plasma, blood, or urine samples. Plasma protein binding of the carboxylate was 90% (range 80%-96%) vs. 95% (range 90%-98%) for the lactone. Conclusions: AR-67 is a lipophilic camptothecin with a unique PK profile. Unlike other clinically approved analogs with lower lactone stability (35%-65%), over 85% of the AR-67 AUC is in the active lactone form. This high lactone-low carboxylate exposure coupled with the apparently limited metabolism of AR-67 may result in increased activity and decreased toxicity.

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