Management of Levonorgestrel and Etonogestrel Adsorption to Polypropylene During in Vitro Release from Long-Acting Release Dosage Forms

Adsorption of active pharmaceutical ingredients (APIs) to various surfaces of labware can skew data during drug development. We observed substantial loss of two clinically significant hydrophobic contraceptive steroids, levonorgestrel (LNG) and etonogestrel (ENG), during in vitro release evaluation from poly(lactic-<i>co</i>-glycolic acid) (PLGA) microspheres and microneedles, leading to incomplete drug recovery. Our investigation identified the adsorption of these APIs to polypropylene (PP) as a key factor adversely affecting both the drug loss and sensitivity/linearity of ultra performance liquid chromatography (UPLC)-ultraviolet (UV) assays during in vitro release tests. Furthermore, LC-MS/MS plasma assays showed improved accuracy and reduced variability when calibration curves were prepared in glass vials instead of in PP vials. Adsorption was also observed in simple PP pipet tips in standard aqueous solutions. To address the adsorption and mass loss, we found that adding acetonitrile to samples after they were collected from glass in vitro release vessels effectively mitigated mass loss. The presence of plasticizers, such as diisononyl adipate found in common porous nylon bags used to contain PLGA dosage forms, was shown to reduce LNG/ENG adsorption to PP. In addition, atomic force microscopy provided experimental characterization of the drug solution-PP/glass interface, showing a higher surface area of the PP surface from nanostructures and an increase in adhesion force with LNG molecules. A thorough understanding and mitigation of API adsorption, including selection of appropriate apparatus materials and considering the possible role of contamination from plasticizers and other leachables, are important for the accurate development of analytical assays supporting pharmaceutical development of long-acting contraceptives.