Use of Partial Area Under the Curve in Bioavailability or Bioequivalence Assessments: A Regulatory Perspective

The role of partial area under the curve and maximum concentrations in assessing the bioequivalence of long-acting injectable formulation of exenatide_A sensitivity analysis

To ensure therapeutic equivalence between the long-acting injectable (LAI) products, additional PK metrics other than Cmax and AUC were considered necessary. However, regarding the selection of additional PK metrics for bioequivalence (BE) assessment of exenatide LAI, a discrepancy existed between EMA's and USFDA's product-specific guidance. The EMA recommends that both the maximum plasma concentration in the initial-release phase (Cmax,1) and the extended-release phase (Cmax,2) should be determined. Nevertheless, the USFDA recommends the use of the partial area under the curve (i.e., the area under the curve from week 4 to the last sampling point; pAUC4w-t). The focus of this study was to compare the sensitivity of different PK metrics, including Cmax,1, Cmax,2, pAUC4w-t, early and late pAUC metrics truncated at different time points (three, four, five, six and seven weeks), to formulation-related parameters and pharmacodynamic (PD) markers of glycemic control. A sensitivity analysis was conducted using the published PK/PD model of exenatide LAI. The results indicated that Cmax,1 and Cmax,2 exhibited comparable sensitivities. AUC4w-t was sensitive to changes in detecting the differences in formulation-related parameters and PD markers of glycemic control, but did not provide superior sensitivity performance compared to Cmax,1 and Cmax,2. Among all tested PK metrics, AUC7w-t was found to be the most sensitive. The optimal cut-off time point for the pAUC should be set at the time of maximum plasma concentration in the extended-release phase (approximately 6-7 weeks). These results may provide useful insights into the selection of appropriate PK metrics for BE determination of exenatide LAI.

In Vivo Characterization of Perseris and Compositionally Equivalent Formulations

Perseris is asubcutaneous extended-release risperidone in situ forming implant (suspension) indicated for the treatment of adult schizophrenia. Owing to the release rate controlling polymer poly(lactide-co-glycolide) (PLGA), one injection of Perseris can deliver risperidone for one month, which significantly reduces the administration frequency and improves patient compliance. The PLGA and drug used in Perseris was previously identified through reverse engineering and two compositionally equivalent formulations (F-1 and F-2) showing similar in vitro drug release were developed. The current work focuses on in vivo exploration of Perseris and the developed compositionally equivalent formulations using a rabbit model and further evaluate the sameness of the developed formulations compared to Perseris. The in vivo pharmacokinetic (PK) profiles, drug absorption rate, phase separation rate, macro appearance, weight loss as well as the water uptake of the solidified drug depots at different time points were investigated and compared with the in vitro release data as well as with dog and human in vivo data available in literature. Results show that the rabbit PK profile of Perseris was relevant with those obtained from both the dog model and the clinical data, indicating that the rabbit model is appropriate for investigation of the in vivo performance of risperidone implants. Consistent with their similar in vitro drug release, the two compositionally equivalent formulations demonstrated similar PK profiles, drug absorption rates, weight loss and swelling in vivo compared to Perseris. Although the erosion mechanism appeared to be similar between in vitro and in vivo, there were in vitro-in vivo differences concerning the drug release kinetics, phase separation rates and swelling behavior. This work provides a comprehensive in vitro/in vivo understanding of Perseris and the developed compositionally equivalent formulations, which will be beneficial for future development of generic as well as novel PLGA in situ forming implant products.