Retargeting of adenoviral infection to melanoma: Combining genetic ablation of native tropism with a recombinant bispecific single-chain diabody (scDb) adapter that binds to fiber knob and HMWMAA

Gene therapy is an emerging and promising modality for the treatment of malignant melanoma and other neoplasms for which conventional therapies are inadequate. Various therapeutic genes have shown promise for tumor cell killing. However, successful gene therapy depends on the development of efficient and targeted gene transfer vectors. Here we describe a novel strategy for targeting of adenovirus-mediated gene transfer to melanoma cells. This strategy combines genetic ablation of native adenoviral tropism with redirected viral binding to melanoma cells via a bispecific adapter molecule, a bacterially expressed single-chain diabody, scDb MelAd, that binds to both the adenoviral fiber protein and to the high molecular weight melanoma-associated antigen (HMWMAA). This antigen is widely and specifically expressed on the surface of melanoma cells and its expression is associated with tumor development and progression. Our results showed specific and strong binding of the anti-HMWMAA scFv RAFT3 and the bispecific adapter scDb MelAd to melanoma cells. In adenoviral infection experiments, we demonstrated i) substantially (>50-fold) reduced infectivity of capsid mutant adenoviruses, ii) restored (up to 367-fold increase), CAR-independent and HMWMAA-mediated infectivity of these mutant viruses by scDb MelAd specifically in melanoma cells, and iii) higher levels of transgene expression in melanoma cells by fiber mutant virus complexed with scDbMelAd, relative to a vector with wildtype fibers. We confirmed the utility of this targeting strategy with human primary melanoma cells that represent clinically relevant substrates. These experiments established that the retargeting strategy mediates up to 54-fold increased adenoviral gene transfer to CAR-negative melanoma cells compared to the vector with native tropism. Hence, the HMWMAA-targeted adenoviral vector lacking native tropism exhibits both enhanced specificity and augmented infectivity of gene transfer to melanoma cells, suggesting that it is feasible to use this vector to improve gene therapy for malignant melanoma. (C) 2003 Wiley-Liss, Inc.