Abstract
Background: Vincristine sulfate (VCR) is a cell-cycle specific, lipophilic, anti-cancer drug that inhibits cell division by binding to tubulin in mitotic spindles. Marqibo is a proprietary sphingomyelin/cholesterol liposome (OPTISOME) encapsulated formulation of VCR with an extended circulating half-life and the potential for preferential malignancy targeting, exposure, and anti-cancer activity. This report evaluates the concentration and accumulation of VCR in solid tumor tissue as well as tissues frequently involved with lymphoid malignancies in tumor-bearing mice following equivalent doses of Marqibo or conventional VCR.
Methods: Mice were implanted subcutaneously with MX-1 human breast tumor tissue. When the tumor volume reached 162–485 mm3, mice received a single intravenous dose of Marqibo, containing 1.5 mg/kg of [14C]-VCR, or 1.5 mg/kg of non-liposomal [14C]-VCR. The total radioactivity from parent compound and metabolites in tissue, VCReq, was analyzed by Quantitative Whole Body Autoradioluminography (QWBA) and tissue digestion (TD). QWBA analysis of bone marrow (BM), tumor tissue, lymph nodes, liver, and spleen required 1 flash-frozen mouse per group at various time intervals. Sagittal sections were examined histologically for radioactivity. For TD, all tissues except bone marrow were obtained from 3 mice per group prior to infusion and at selected time points. The samples were chemically digested and analyzed for radioactivity by liquid scintillation counting.
Results: QWBA analysis revealed at least two-fold higher VCReq tissue concentrations in the Marqibo-injected mice compared to the conventional VCR-injected mice.
VCReq Tissue Concentration (mcg/g) at 48 hours Post-Injection BM Tumor Lymph Liver Spleen Marqibo 0.99 1.35 1.98 0.47 6.08 VCR 0.36 0.31 -- 0.13 0.76 Marqibo/VCR ratio 2.75 4.35 -- 3.61 8
The TD results are consistent with the QWBA results shown in the table. Marqibo-injected mice demonstrated a minimum 5-fold increase in VCR exposure over a 96 hour period (VCReq AUC analysis) compared to VCR-injected mice over a 48 hour period, as defined in the parameters of the study, in the implanted tumor (123 vs. 21 hr*μg/g), lymph nodes (121 vs. 16.5 hr*μg/g), liver (68.8 vs. 12 hr*μg/g), and spleen (512 vs. 68.9 hr*μg/g). Though the parameters for the AUC values may favor Marqibo, the Tmax of all specified tissues are at least 16 times greater in Marqibo-injected mice than in VCR-injected mice (16 hrs vs. 1hr in tumor, spleen, and lymph; 4 hrs vs. 15 min. in liver; 16 hrs. vs. 15 min. in the kidney). The greatest increase in VCR exposure produced by Marqibo was observed in the spleen and lymph nodes (7.4- and 7.3-fold).
Conclusions: Marqibo resulted in targeted delivery of VCR; concentration of VCR in tumor tissue, BM, lymph nodes, liver, and spleen; and maintenance of significant tissue drug concentrations for an extended period of time compared to conventional VCR. The ability of Marqibo to target drug to these tissues and organs makes it particularly attractive as a treatment for hematologic malignancies like myeloma, lymphoma, and acute lymphoblastic leukemia (ALL). Late-stage clinical development of Marqibo in adult ALL is underway.
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