Favorable Interactions between Aplidin (Plitidepsin) and Conventional Anticancer Agents in Leukemic Cell Lines and Acute Leukemia Patient Samples.
Authors: Kaspers Gertjan J. , Broekhuizen R. , Goemans B. , Vladimirskaya E. , Jimeno J. 
Published: October 2019

Abstract
Aplidin (Plitidepsin, APLD) is a novel depsipeptide, without clinically relevant myelosuppression. APLD is in phase II trials in adults, and in phase I in children. We have reported in vitro antileukemic activity of APLD at picomolar (clinically achievable) concentrations. APLD was more cytotoxic towards both ALL and AML cells than to normal bone marrow (BM) samples (Bresters et al., Leukemia 2003). Moreover, relapsed ALL samples were similarly sensitive to APLD as initial ALL samples. We now studied interactions between APLD and conventional agents in the myeloid leukemia cell lines HL60, U937 and Kasumi, and the lymphoblastic leukemia cell lines CEM, Jurkat, and 697. We also tested 10 AML and 9 ALL patient samples, and 5 normal BM samples. APLD was combined with cytarabine (ara-C), daunorubicin (DNR) or etoposide (VP16) in the myeloid cells, and with dexamethasone (DXM), vincristine (VCR) or l-asparaginase (ASP) in the lymphoblastic cells. In normal BM samples APLD was combined with each of the 6 drugs mentioned. For the cell lines 3 schedules were tested: pre-incubation: 24 hours exposure to APLD followed by 48 hours of co-incubation; co-incubation for 72 hours; post-incubation: 24 hours pre-exposure to conventional agent followed by 48 hours of co-incubation. For the patient samples, only co-incubations were done. Inhibition of cell proliferation and cytotoxicity was measured using the colorimetric MTT assay. Results were expressed as combination indices (C.I.), calculated with Calcusyn software. C.I. <1, 1 and >1 indicate synergism, additivity and antagonism, respectively. In all lymphoblastic cell lines, synergism was consistently observed between APLD and the other drugs in case of post-incubation (C.I. 0.5–1.0). Overall, synergism in >50% of all schedules occurred with ASP and DXM. In all 3 myeloid cell lines, synergism was again consistently observed between APLD and the other drugs in case in post-incubation (C.I. 0.8–0.9), except for the combination with DNR in U937 (C.I. of 1.2). Overall, synergism in >50% of all schedules occurred with ara-C. In patient samples, large differences in drug interactions were observed between samples, drugs and specific combinations and concentrations. In AML, interactions between APLD and DNR and VP16 were mainly additive or antagonistic. However, APLD with ara-C was often additive or synergistic, with a median C.I. of 0.7–1.0 for 6 out of 12 combinations tested. In ALL, most interactions between APLD and DXM, VCR and ASP were additive or antagonistic, with some synergistic interactions with VCR. In normal BM samples, the majority of interactions was again additive or antagonistic. In conclusion, we found synergistic interactions between APLD and conventional agents in both myeloid and lymphoblastic leukemia cell lines, especially in case of post-incubation (76% synergistic plus 12% additive interactions). In patient samples, such synergism was less pronounced, although additive interactions were common and synergism was observed especially between APLD and ara-C in AML. Thus, clinical studies in ALL should focus on combining APLD with ASP and/or DXM. In AML, the combination of APLD with ara-C is of interest. In both diseases, a schedule with administration of the conventional agent followed by APLD seems warranted. Underlying mechanisms of the variety of drug interactions that we observed are unknown and require additional studies.