Gene therapy for erectile dysfunction

The past decade has seen an explosion of new information on the physiology of penile erection, pathophysiology of erectile dysfunction (ED), and development of new oral agents (e.g., three PDE5 inhibitors) to manage ED. Although all three selective PDE5 inhibitors are effective in the majority of ED cases, these oral medications have failed in certain disease states, such as diabetic ED, postprostatectomy ED, and severe veno-occlusive dysfunction. Only about 50% to 60% of these cases benefit from PDE5 inhibitor therapy, prompting the development of new approaches, including gene-based therapies for the treatment of ED. The penis is a convenient tissue target for gene therapy because of its external location and accessibility, the ubiquity of endothelial lined spaces, and low level of blood flow, especially in the flaccid state. Initially, gene therapy has been reserved for the treatment of life-threatening disorders including cancer, hereditary and acquired diseases. However, gene therapy is an attractive therapeutic possibility for the treatment of ED. Evolution of nitric oxide (NO), a small gaseous, lipophilic signaling molecule that is produced by nitric oxide synthase (NOS) activates guanylate cyclase (GC), resulting in increased cyclic guanosine monophosphate (cGMP) production, plays a significant role in our understanding of cavernosal smooth muscle physiology. Many gene therapy strategies have focused on the NO/GS/cGMP pathway. All three NOS isoforms, endothelial NOS (eNOS), neuronal NOS (nNOS), and iNOS have been used for gene therapy in order to modulate erectile response. Various viral and nonviral vectors have been used to date for the transfer of genetic material to the target cell or tissues with various degrees of success. Recently, second generation or "gutless" (helper-dependent) adenovirus vectors have been developed in order to reduce cellular toxicity and immune response, while increasing efficient gene therapy. Varieties of other gene therapy trials have also been undertaken for the treatment of ED and are the focus of this review.