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6 General discussion and Future Prospects | 147 Another NK cell activating product is ENKASTIM-ev, developed by Multimmune GmbH, which mimics the functions of heat shock protein 70 (Hsp70). ENKASTIM-ev resulted in NK specific activation and actively targeted Hsp70 expressing tumors. Safety of Hsp70 activated autologous NK cells has been documented in a phase I study in patients with metastatic colorectal and non-small cell lung cancer 26 . Enhancing NK cell homing functions Gamida-Cell developed a feeder cell free NK cell culture and expansion system containing nicotinamide (NAM) to generate NK cells from PBMC apheresis products. Nicotinamide, a derivative of vitamin B3, serves as a potent inhibitor of NAD dependent enzymes. Results from in vivo studies in mice showed that PBNK cells expanded with NAM in feeder free cultures exhibited increased homing potential towards lymphoid organs, with a significant increase in the expression of CD62L (L-selectin) compared to cultures without NAM 27 . Genetic modification of NK cells In addition to successful expansion, differentiation and demonstrable anti-tumor effects of NK cells, NK cell tumor targeting can be made more specific by employing chimeric antigen receptors (CARs) as demonstrated for T cell adoptive transfer strategies 28 . CARs are recombinant Ab-based molecules that upon expression in immune effector cells bind antigens of interest on target cells, resulting in immune activation and enhanced immune effector cell survival through specific intracellular signalling motifs fused to the antigen binding domain (usually a single-chain Fv fragment [scFv]). PBNK-CARs against breast cancer (HER-2), neuroblastoma (CD244) and CD19+ B-cell precursor cell ALL (CD19) 29 have demonstrated efficacy in preclinical studies, while two clinical trials are ongoing using modified haplo-identical PBNK cells with anti-CD19 CARs in B cell malignancies (NCT00995137 and NCT01974479) 28 . NantKwest, is actively involved in enhancing the functions of its lead product, parental NK-92 cells (activated NK cells, aNK), through gene modifications employing CARs to make them target specific. NK-92 CARs (taNK) are developed against tumor markers in neuroblastoma (GD2), melanoma (GPA7) 30 , breast cancer (EpCAM, HER-2, EGFR) 31,32 , multiple myeloma (CS1 33 , CD138 34 ) and leukemias (CD19, CD20) 35 and have shown efficacy in preclinical studies. In an alternative approach NK-92 cells have also been modified to express CD16a (high affinity NK cells, haNK) to promote ADCC 36 . NantKwest has also partnered with Sorrento Therapeutics to develop NK-92 CARs targeting programmed death-ligand1 (PD-L1) 37 and receptor tyrosine kinase like orphan receptor 1 (ROR-1) 38 . Besides specific targeting of tumor antigens and strategies to promote ADCC, Nkarta therapeutics developed NKG2D CARs (NKG2D- CD3ζ - DAP10) using NK-92 cells and PBNK cells, which exhibited enhanced cytotoxicity against osteosarcoma and hepatocellular