Evaluation of In-Use Physicochemical Stability of Diluted Isatuximab (ISA)

Introduction
Isatuximab (SARCLISA^®) is a chimeric IgG1 monoclonal antibody targeting CD38, expressed on the surface of both normal and tumor hematopoietic cells. It is approved for the treatment of multiple myeloma. Current stability data restrict the use of reconstituted and diluted solutions to 48 hours, limiting the possibility of preparing doses in advance in hospital pharmacies. This constraint not only hinders workflow optimization but also contributes to economic and environmental waste. The objective of this study is to assess the physicochemical stability of isatuximab stored at 4 °C for up to 28 days, with the aim of extending its in-use shelf life and improving pharmaceutical management.

Materials et Methods
Forced degradation tests (extreme pH, oxidation, temperature, agitation) were conducted to validate the robustness of analytical methods. Samples at 1 and 3 mg/mL were stored at 4°C. The physicochemical stability of ISA was evaluated over 28 days using complementary analytical methods: size-exclusion chromatography coupled with dynamic light scattering (SEC-DLS) for aggregates, SDS-PAGE analysis after tryptic digestion (to detect structural modifications), cation exchange chromatography (CEX) to assess charge variants, and hydrophobic interaction chromatography (HIC) for monitoring potential conformational changes. FcR affinity and protein A binding assays were also performed to assess respectively biological activity and quantification. Biological activity was evaluated by Fc receptor (FcR) affinity assays, while protein A binding assays were used for quantification.

Results
The methods were developed and validated in accordance with ICH Q2(R1) guidelines. Oxidative and alkaline stress (pH 8–10) led to notable degradation, validating method sensitivity. No significant alteration was observed under agitation or at 37°C. SDS-PAGE revealed oligomer formation (non-migrating band) after 2 hours at 60°C, with no changes at 4°C or 30°C. Under standard conditions, no major variation in concentration (<10%), no appearance of aggregates or degradation products was detected up to Day 28 at 4°C, and biological activity was maintained.

Conclusion
Isatuximab maintained its physicochemical stability for up to 28 days at 4 °C, supporting the possibility of extending its in-use shelf life. Such an extension could offer significant organizational, economic, and environmental advantages for hospital pharmacies. However, its susceptibility to extreme conditions underscores the need for strict compliance with recommended storage practices.