Biological therapies play an increasing role in cancer treatment, although the number of naked antibodies showing clinical efficacy as single agent remains limited. One way to enhance therapeutic potential of antibodies is to conjugate them to small molecule drugs. This combination is expected to bring together the benefits of highly potent drugs on the one hand and selective binders of specific tumor antigens on the other hand. However, designing an ADC is more complex than a simple meccano game, requiring thoughtful combination of antibody, linker, and drugs in the context of a target and a defined cancer indication. Lessons learned from the first-generation antibody-drug conjugate (ADC) and improvement of the technology guided the design of improved compounds which are now in clinical trials. Brentuximab vedotin (Adcetris(®)), an anti-CD30 antibody conjugated to a potent microtubule inhibitor for the treatment of Hodgkin's lymphoma and anaplastic large cell lymphomas, is the only marketed ADC today. A total of 27 ADC are currently undergoing clinical trials in both hematological malignancies and solid tumor indications. Among them, T-DM1 (trastuzumab emtansine), an ADC comprised of trastuzumab conjugated to DM1, via a non-cleavable linker, is showing very promising results in phase III for the treatment of HER2-positive refractory/relapsed metastatic breast cancer. Other compounds, such as CMC-544, SAR3419, CDX-011, PSMA-ADC, BT-062, and IMGN901 currently in clinical trials, targeting varied antigens and bearing different linker and drugs, contribute to the learning curve of ADC, as do the discontinued ADC. Current challenges include improvement of the therapeutic index, linked to a careful selection of the targets, a better understanding of ADC mechanism of action, the management and understanding of ADC off-target toxicities, as well as the selection of appropriate clinical settings (patient selection, dosing regimen) where these molecules can bring highest clinical benefit.

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Purpose:To develop and compare three copper 64 ( 64 Cu)-labeled antibody fragments derived from a CA6-targeting antibody (huDS6) as immuno-positron emission tomography (immuno-PET)-based companion diagnostic agents for an antibody-drug conjugate by using huDS6. Materials and Methods:Three antibody fragments derived from huDS6 were produced, purified, conjugated to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and evaluated in the following ways: (a) the affinity of the fragments and the DOTA conjugates was measured via flow cytometry, (b) the stability of the labeled fragments was determined ex vivo in human serum over 24 hours, and (c) comparison of the in vivo imaging potential of the fragments was evaluated in mice bearing subcutaneous CA6-positive and CA6-negative xenografts by using serial PET imaging and biodistribution. Isotype controls with antilysozyme and anti-DM4 B-Fabs and blocking experiments with an excess of either B-Fab or huDS6 were used to determine the extent of the antibody fragment 64 Cu-DOTA-B-Fab binding specificity. Immunoreactivity and tracer kinetics were evaluated by using cellular uptake and 48-hour imaging experiments, respectively. Statistical analyses were performed by using t tests, one-way analysis of variance, and Wilcoxon and Mann-Whitney tests. Results:The antibody fragment 64Cu-DOTA-B-Fab was more than 95% stable after 24 hours in human serum, had an immunoreactivity of more than 70%, and allowed differentiation between CA6-positive and CA6-negative tumors in vivo as early as 6 hours after injection, with a 1.7-fold uptake ratio between tumors. Isotype and blocking studies experiments showed tracer-specific uptake in antigen-positive tumors, despite some nonspecific uptake in both tumor models. Conclusion:Three antibody fragments were produced and examined as potential companion diagnostic agents. 64 Cu-DOTA-BFab is a stable and effective immuno-PET tracer for CA6 imaging in vivo.q RSNA, 2015

Cryptophycins are a class of macrocyclic depsipeptides produced as secondary metabolites by cyanobacteria of the genus Nostoc and were found to bind to microtubules at the vinca site. The natural product C-1 isolated in 1990 and the synthetic derivative C-52 displayed potent in vitro and in vivo antitumor activity in preclinical models. Cryptophycin C-52 (LY355703) produced marginal antitumor activity at MTD in two phase II lung cancer trials and was therefore discontinued. Considering its higher potency versus other tubulin binders such as maytansine and auristatin, this chemical series was selected for an ADC approach. Several conjugates were evaluated in vitro and in vivo based on the conjugation of C52 derivatized at the para-benzylic position of the macrocycle. As described in this study, this optimization led to the discovery of cryptophycin ADC which displayed potent antitumor activity in vivo. However, these conjugates were found unstable in mice plasma while being stable in the plasma of non-rodent species. This species-dependent instability was shown to be the result of a mice plasmatic metabolization of C52 macrocycle once conjugated to the antibody. We therefore designed and synthesized new cryptophycin ADC which exhibited improved plasmatic stability when tested in mice and enhanced therapeutic index in comparison with C52 ADC. In conclusion, these data demonstrated the potent in vitro and in vivo antitumor activity of these new cryptophycin ADC and warrant further development of this cytotoxic payload for an ADC approach. Citation Format: Marie-Priscille Brun, Hervé Bouchard, François Clerc, Jidong Zhang, Pierre-Yves Abecassis, Céline Amara, Eric Beys, Florence Efremenko, Céline Nicolazzi, Marie-Hélène Pascual, Olivier Pasquier, Alain Krick, Pierre-François Berne, Laurence Gauzy, Nathalie Karst, Sylvain Huille, Christophe Henry, Anne-Marie Lefebvre, Ingrid Sassoon. Towards new cryptophycins as promising payloads for ADC. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-053.

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