Vincristine Sulfate Liposome Injection (VSLI, Marqibo®) Facilitates Increased Delivery of Vincristine Sulfate to Target Cancer Tissues.

Standard vincristine sulfate (VCR) plasma pharmacokinetics (PK) is described by a bi-exponential profile with a very short half-life followed by a longer elimination half-life; the volume of distribution is large, suggesting wide and diffuse distribution and perhaps tissue binding. These PK characteristics may limit optimal therapeutic activity of VCR by limiting C_maxand drug exposure in target tissues involved with cancer. VinCRIStine sulfate LIPOSOME injection (VSLI, Marqibo) is a sphingomyelin- and cholesterol-based nanoparticle formulation of VCR that was designed to be different from and overcome the dosing and pharmacokinetic limitations of standard VCR. The objectives for development of VSLI were to: 1) increase the plasma circulation time; 2) increase tumor tissue delivery by preferential extravasation from fenestrated (“leaky”) vasculature; 3) accumulation in tumor tissues; and 4) slow release of VCR in tumor tissues instead of the systemic circulation. VSLI recently received accelerated FDA approval for use in adults with advanced, relapsed and refractory Philadelphia chromosome negative ALL and is in development for pediatric ALL and aggressive NHL.

The PK and tissue distribution of VCR was evaluated in Sprague-Dawley rats following a single IV bolus dose (2.0 mg/m²) of VSLI or VCR, utilizing ³H-VCR as a marker. The tissue levels of total VCR were measured for up to 72 hours post-dose. PK analysis was performed using noncomparmental methods with the WinNonlin software package.

The PK profile and calculated parameters of VSLI in rats showed a substantially lower total vincristine clearance (CL) and volume of distribution (V_dz) and correspondingly greater area under the plasma concentration versus time curve (AUC) compared to standard VCR. VSLI has a long circulation time and remains in the plasma instead of being widely distributed in tissues. For most tissues from VSLI treated animals, tissue to plasma concentration ratios increased over time and peaked at 72 h after VSLI injection, indicating progressive accumulation of radiolabeled drug from plasma into the tissues. The rank order of tissues based on C_max demonstrated that total VCR concentrations in MPS tissues (i.e., spleen, liver, lymph nodes and bone marrow) and in ovaries were substantially higher than in other organs or tissues. The lowest radioactivity levels were observed in brain, spinal cord, nerves and muscle. Higher exposures, as measured by AUC_inf, of VCR were observed in plasma (83 fold), spleen (12 fold), lymph nodes (10 fold), liver (4 fold) and bone marrow (2 fold) following a radiolabeled dose of VSLI compared to VCR.

The long circulation time and small, 100 nm, mean nanoparticle size of VSLI facilitate extravasation from fenestrated vasculature and accumulation in tissues involved in hematologic malignancies such as lymph nodes, bone marrow and spleen. Slow release of VCR in target tissues following administration of VSLI results in higher and prolonged tissue drug levels providing superior drug delivery to tissues than the same doses of standard formulation of VCR.

Silverman:Talon Therapeutics: Employment. Deitcher:Talon Therapeutics: Employment, Equity Ownership.