A Spoonful of Safety - Demand in the pharmaceutical industry has sharply increased for the manufacture of targeted and potent drugs that improve efficacy and reduce side effects, such as antibody-drug conjugates (ADCs). By combining a monoclonal antibody (mAb) and a highly potent small molecule toxin (usually a cytotoxic compound), ADCs directly target cancerous cells.

In the body, the mAb specifically binds to an antigen on the outer surface of cancerous cells and the ADC is absorbed inside the cell via endocytosis; the cleavage of the linker (that is designed to be labile within the cell medium) releases the toxic substance, leading to cellular death. This therapy is very promising because of its high selectivity and significantly reduced side effects. Indeed, theoretically, healthy cells should not be affected by the treatment, and that is of upmost importance for the patient. Lower dosage can be used since the treatment is delivered only where needed. In standard (nontargeted) chemotherapy treatment, an acceptable therapeutic window has to be found for the medication to affect malign cells with a minimal impact on healthy cells; hence molecules exhibiting particularly high cytotoxicity have to be discarded. Targeted therapies enable a safe use of these same molecules as toxins or ADC "payloads." However, from a manufacturing point of view, these ADC toxins require higher containment levels than typical highly potent active pharmaceutical ingredients (HPAPIs) to ensure a safe production environment for the operators while preventing product contamination and protecting the environment.

The main challenge in HPAPI manufacturing resides in the ability of the producer to operate safely and adapt to the constraints of innovative and ever more potent molecules. It implies using an appropriate confinement strategy based on engineering controls, collective and personal protection equipment, continuous training for all personnel on site, establishment of standard operating procedures (SOP), monitoring occupational hygiene, and implementing medical surveillance of the employees.

Safely Manufacturing Highly Potent Compounds

There are no official guidelines for the control and monitoring of safe handling of HPAPIs, which is left to the companies' appreciation. Therefore, this mastery can be obtained only through an extensive and long-lasting experience in the area and only a few manufacturers hold the appropriate expertise to handle these compounds. External assessment of HPAPI production safety is provided by specialized companies such as SafeBridge Consultants Inc. The company's certification is the only independent evaluation system to date and assesses the management, the evaluation, the containment, the control, and the communication elements of HPAPI operations.

When it comes to HPAPI manufacturing, finding a partner operating SafeBridge certified facilities ensures that a high level of mastery of such manufacturing constraints has been demonstrated. To date, only 11 companies have met the consultants' current industrial standards (source: SafeBridge Consultants Inc., February 2012). SafeBridge has established a potency rating system in which compounds are classified into four categories depending on their Occupational Exposure Limit (OEL), the maximal exposure acceptable for employees over an eight-hour work shift. For each compound, an exhaustive potency assessment has to be carried out before any work starts, and the highly potent compounds are classified in category 3 (OEL 10 µg.m-3.8h-1) and category 4 (OEL 30 ng.m-3.8h-1).

As the development of ADCs enables the therapeutic use of more potent molecules, a shift to lower OEL is currently observed for several new molecules of interest, and ever-higher levels of containment have to be met. While the certification applies to specific areas within a factory, these usually include a broad range of facilities such as R&D laboratories, kilo labs, and high containment manufacturing areas and their associated quality control laboratories. Indeed, all areas where HPAPIs are handled, including QC laboratories, have to comply with the appropriate confinement level. The outsourcing of HPAPI analysis is not common, and most analyses should preferably be performed in house. In addition, the high level of purity requires adequate detection and rejection of highly potent impurities, which requires the manufacturer to operate highly effective analytical equipment and measurement methods. Frequent hygiene monitoring is adequate to ensure that the necessary containment is achieved at all times, and all operators involved in HPAPI manufacturing undergo regular health checks.

Reaching High Purity in Confined Environment

Advanced purification methods are required to deliver active ingredients at the desired level of purity. Efficient purification at large scale is often key in getting cost-efficient access to semi-synthetic starting materials from various sources including fermentation broths and natural extracts. Two factors imply the use of advanced purification technologies for the production of HPAPIs: 1) their intrinsic complexity (whether these molecules are extracted from biomass or synthesized from scratch) resulting in mixtures containing very closely related impurities, and 2) the very high level of purity typically required. Preparative chromatography is almost a must and, from an economical point of view, preparative HPLC is particularly well adapted to reaching high purity for moderate scale productions (rarely exceeding a few hundred kilograms per year at production scale for HPAPIs, with even smaller volumes for ADC toxins). The main advantage of preparative HPLC is the very predictive assessment of the results on large scale, based on automated screening results obtained from just a few experiments and sub-gram amounts of material, due to a direct scalability. As a consequence, an accurate estimation of productivity and costs at large scale is obtained very early in the process development phase, while minimizing the handling of the highly potent compound. In addition, advanced computer simulation tools allow for a rapid, reliable development and straightforward validation of the process. Furthermore, compared with traditional low-pressure chromatography, preparative HPLC offers considerably improved unit productivity and therefore a smaller system and much less stationary phase are required. As a consequence, the column can be easily located in a confined area and the handling of the packing material and the cleaning of the system are much easier. Preparative HPLC has proven to be robust and reliable at commercial scale. It constitutes the purification method of choice for the production of commercial HPAPIs and ADC toxins in particular.

Antibody-drug conjugates and other types of vectorization of toxins and HPAPIs require specific technologies depending on the type of payload (maytansinoids, auristatins, anthracyclins, pyrrolobenzodiazepines, calicheamicins, taxanes, etc.) and linker/vector used (amino acids and peptides, pegylated and polymeric chains, other complex side chains). Innovative and tailored solutions for downstream purification and isolation may involve low-pressure chromatography (affinity chromatography, ion exchange, gel permeation, etc.), tangential flow (ultrafiltration) technologies that have to be operated according to biopharmaceutical and HPAPI standards. Since the cytotoxic payloads are usually complex and fragile molecules presenting several chiral centers, they require mild and efficient technologies. Tangential flow filtration (TFF) is well suited for small molecules isolation, concentration and for formulation of large molecule APIs. As for HPLC, the limited size of TFF equipment is well adapted to implementation in confined areas.

Conclusion

In a world where molecules reaching the market are ever more complex and need to meet very stringent purity standards, the right combination of technologies is more important than ever. The manufacturing of new targeted drugs, such as ADCs, fully benefits from the mastery of synthesis and purification technologies. Since these molecules lay at the interface of chemical and biological domains, their development and manufacturing fully benefit from the cross fertilization occurring only in CMOs with expertise in both areas. When it comes to ADC toxins, a careful choice of the manufacturing partner is key to success.