Microdosing Framework for Absolute Bioavailability Assessment of Poorly Soluble Drugs: A Case Study on Cold-Labeled Venetoclax, from Chemistry to the Clinic.

This work presents an end-to-end approach for assessing the absolute bioavailability of highly hydrophobic, poorly water-soluble compounds that exhibit high non-specific binding using venetoclax as a model drug. The approach utilizes a stable labeled intravenous (IV) microdose and requires fewer resources compared to traditional approaches that use radioactive 14 C-labeled compounds. The stable labeled venetoclax and internal standard were synthesized, then an IV formulation was developed. In the clinical study, female subjects received a single oral dose of venetoclax 100mg followed by a 100-µg IV dose of cold-labeled 13 C-venetoclax at the oral Tmax . The IV microdose was prepared as an extemporaneous, sterile compounded solution on the dosing day by pharmacists at the clinical site. Several measures were taken to ensure the sterility and safety of the IV preparation. A sensitive LC-MS/MS method was developed to allow the detection of plasma levels from the IV microdose. Plasma samples were collected through 72h, and pharmacokinetic parameters were estimated using non-compartmental methods. Post-dosing sample analysis demonstrated the consistency of the preparations and allows the precise calculation of the pharmacokinetic parameters based on the actual injected dose. The absolute bioavailability of venetoclax was estimated at 5.4% under fasting conditions. Venetoclax extraction ratio was estimated to be 0.06 suggesting that the fraction transferred from the enterocytes into the liver is limiting venetoclax bioavailability. The proposed framework can be applied to other highly hydrophobic, poorly water-soluble compounds that exhibit high non-specific binding to support the understanding of their absorption and disposition mechanisms and support formulation development.