Biorelevant in vitro release/dissolution tests are designed to predict the in vivo behavior of a drug and are crucial in understanding its in vivo performance. Currently, there is no standardized compendial in vitro release testing methods or regulatory guidance’s for release/dissolution testing of implants due to their complex physiological locations.Furthermore, existing compendial methods do not capture the local release profile of ‘novel’ parenterals in physiological low fluid volume surrounding areas. Long acting and in situ forming implants with orthobiologic proteins and peptides have increased over the past few decades due to a better understanding of genetic engineering. One of these products, INFUSE® Bone Graft (Medtronics, MN, USA), is an implant which helps in bone regeneration at the trauma site and is comprised of a) an absorbable collagen sponge (ACS) and b) recombinant human bone morphogenetic protein-2 (rhBMP-2). INFUSE® Bone Graft is an FDA approved product for acute, open shaft tibial fractures, lumbar spinal fusions and sinus or ridge augmentations in the jaws. The evaluation of implant products such as INFUSE® Bone Graft requires a good understanding of local and systemic release in vivo in order to ensure safe, effective, and predictable product performance. The primary goal of this study is to develop a predictive ‘biorelevant’ release model, which factors in clinically relevant physiological parameters suitable for studying and effectively predicting extended release of implants, using INFUSE Bone Graft® as our model implant. A novel biorelevant in vitro model was designed and tested. The model was observed to be discriminatory between two different carrier formulations of rhBMP-2 using a model independent approach - similarity factor (f2). Additionally, a high throughput assay to quantify rhBMP-2 release using high performance liquid chromatography with UV/VIS detection was also developed and validated. Successful completion of this study facilitated an in vitro release study design that incorporated the complex biorelevant parameters of implant dosage forms, the model will offer crucial insights into biological performance, and aid in developing methods to characterize release of other similar dosage forms.

In vitro dissolution, release and permeation testing is a common practice during drug product research and development. These in vitro tests, if predictive, are referred to as biorelevant tests and can play diverse roles to facilitate and expedite product development in a cost effective manner. Oral transmucosal products (OTPs) are currently tested using compendial and modified in vitro tests which may or may not be good predictors of in vivo performance due to a lack of biorelevance. A critical need for a broadly applicable and biorelevant in vitro system for OTPs has been expressed in the literature and the goal of this research was the development and validation of a biorelevant in vitro method that can facilitate accurate prediction of the in vivo behavior of OTPs. A combined strategy of appropriate apparatus design and relevant physiological and in vitro variable adjustment was investigated to incorporate biorelevance into evaluation of OTPs. A novel in vitro device, the bidirectional transmucosal apparatus (BTA), was designed and fabricated which allowed simulation of the oral cavity and its physiological variables to evaluate OTPs in a more realistic fashion. The BTA was tested using snus (a type of smokeless tobacco) as the OTP product. A simple and selective high performance liquid chromatographic (HPLC) method with photodiode array (PDA) detection was developed and validated to assess in vitro nicotine release and permeation (Linearity: 0.5 – 32 μg/mL; calibration curve accuracy (%recovery, n=5 ): 97.98-103.20%; calibration curve precision (%RSD, n=5): 0.15-3.14%). The performance of BTA was compared with the modified USP IV flow through apparatus (USP IV) and a commercially available vertical diffusion cell (VDC). The observed in vitro in vivo relationship (IVIVR) slopes with the USP IV, VDC and BTA were 0.27, 2.01 and 2.11 respectively. The BTA was selected over the VDC and USP IV devices because of better simulation and adjustment of variables to incorporate biorelevance in the test of OTPs. Additionally, the BTA allows study of permeation and release simultaneously unlike VDC and USP IV apparatuses. Further, the different BTA parameters were sequentially screened for their impact on in vitro rate of nicotine permeation that can be employed for the optimization of IVIVR for snus. Based on the results, stimulated saliva swallowing rate (SSSR) and media temperature were considered as significant factors affecting in vitro permeation of nicotine and used to further optimize IVIVR for snus. A 32 multifactorial experimental design integrating SSSR (0.32, 1.66 and 3 mL/min) and media temperature (25, 37 and 45 °C) was employed. Based on the response surface analysis, 0.55 mL/min SSSR and 43 °C media temperature were identified as optimal BTA conditions that would give perfect IVIVR (i.e. IVIVR slope close to one) for snus. The experimental value of IVIVR slope (0.92) at these optimal conditions indicated that the BTA is a valid in vitro system for evaluation of OTPs in a biorelevant manner. The applicability of BTA for predicting nicotine permeation from ‘Stonewall’, a dissolvable compressed tobacco was also evaluated. However, comparable in vitro nicotine permeation and in vivo nicotine absorption profiles were not obtained (ratio of in vitro permeation to in vivo absorption rate ranged from 0.04 to 0.14 at different in vitro conditions) either due to the unavailability of reliable clinical data or due to inherently different in vivo behavior of Stonewall compared to snus that would require further modification in the BTA. In conclusion, this research demonstrated the potential of the novel in vitro device to be a valuable tool for the prediction of in vivo performance of snus. The application of the novel bidirectional transmucosal apparatus for other types of OTPs will be an interesting subject for further investigation.