Tumor Targeting Of Alphavirus Vectors

The broad host range of alphaviruses, and particularly their strong preference for neuronal cells, has made cell/tissue specific targeting an especially important issue.

One approach to obtain targeting for SIN has been to introduce IgG binding domains of protein A in the envelope proteins of SIN (37). The study demonstrated that inserts could be introduced in a specific region in the envelope protein E2, which did not affect the viability and infectivity of generated SIN particles. The chimeric SIN particles showed a significantly reduced infectivity could be established through the protein A domain by treatment of cells with a monoclonal antibody against a cell surface protein. In another study, it was demonstrated that a single point mutation in one of the SIN envelope proteins resulted in preferential infection of DCs (38). A recent approach has included the engineering of a fusion between avidin and the low-density lipoprotein (LDL) receptor for targeting biotinylated molecules to desired tissues (39). Studies in vivo demonstrated that malignant rat glioma tumors were transduced by SFV containing such avidin fusion proteins.

Yet another approach to obtain tumor selective transfection was attained by encapsulation of recombinant SFV particles in liposomes (40). By this procedure, targeted gene delivery to human LnCaP prostate tumors implanted in severe combined immunodeficiency (SCID) mice was achieved after systemic delivery of encapsulated SFV-LacZ particles. Systemic administration of encapsulated SFV particles expressing the p40 and p35 subunits of IL-12 to SCID mice with human Panc-1 pancreatic tumors resulted in statistically significant reduction in tumor growth after a single injection (41). Furthermore, an initial phase I study on advanced melanoma and kidney carcinoma patients demonstrated the safe use of this SFV vector in humans. In this study the maximum tolerated dose (MTD) for encapsulated SFV-IL-12 particles was 3 x 109 particles/m2, which might seem relatively low compared with doses for other viruses, usually administered in the range of 1011 particles. However, in this case the dose is mainly dictated by the fever response observed in patients after high IL-12 expression. The IL-12 expression in patients was transient and the IL-12 levels returned to normal levels after 5 to 7 d. Re-administration of encapsulated SFV particles did not induce any immune response, another indication of safe use in humans.

A recent study on the systemic delivery of SIN vectors has generated some unexpected and controversial results (42). Without engineering any targeting sequences on the SIN vectors it was demonstrated that they efficiently homed to tumor cells; intraperitoneal injection of SIN-LacZ virus led to specific expression in implanted BHK tumors. Moreover, strong bioluminescence was observed in animals with BHK tumors, but not in control mice. SIN vectors were also able to target human micrometastatic ES-2 ovarian cancer cells. Stable overexpression of luciferase in ES-2 cells (ES-2/luc) permitted microscopic monitoring of tumor growth. Subcutaneous daily injections of SIN-IL12 virus led to a tumor load of 6.2% of that observed from control animals. Probably the most astonishing example came from studies on spontaneous fibrosarcomas in the mouse tail. Intraperitoneal injections of SIN-luc virus resulted in tumor targeted expression. This tropism to tumor tissue of SIN virus, however, has not yet been studied in full details and it will need additional research of the mechanisms behind it to eventually apply it in clinical settings.

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