Summary

Background: Ion Channel Innovations has developed a gene transfer product, ftMaxi-K, and has begun clinical trials to investigate the effect of increased expression of Maxi-K channels in the smooth muscle of the penis or bladder in patients with erectile dysfunction and those with overactive bladder. The primary function of K channels is to modulate Ca⁺⁺ influx through Ca-channels (i.e., L-type, voltage-dependent). The amount of Ca⁺⁺ that enters the cell through these channels is a major determinant of the free intracellular calcium levels inside the smooth muscle cell, which in turn determines the degree of smooth muscle cell contraction. Increased Maxi-K channel activity is associated with smooth muscle cell relaxation, resulting in, for example, penile erection and detrussor muscle relaxation. A phase I clinical trial that used dMaxi-K has been completed and a similar trial to assess safety of the transfer for overactive bladder is about to begin.

Objectives: To assess the safety and tolerability of escalating dMaxi-K doses by clinical evaluations and laboratory tests, and to measure efficacy objectives by means of the International Index of Erectile Function scale.

Methods: In the erectile dysfunction trial 11 patients with moderate to severe erectile dysfunction were given a single-dose corpus cavernosum injection of dMaxi-K, a "naked" DMA plasmid carrying the human cDNA encoding for the gene for the a, or pore-forming, subunit of the human smooth muscle Maxi-K channel, hSIo. Three patients each were given 500,1000, and 5000 pg and two patients were given 7500 pg doses of ftMaxi-K and followed for 24 weeks. Patient responses were validated by partner responses.

Results: There were no serious adverse events and no dose-related adverse events attributed to gene transfer for any patient at any dose or study visit. No clinically significant changes from baseline were seen in physical evaluations (general and genitourinary), hematology, chemistry and hormone analyses, or in cardiac events evaluated by repeated electrocardiograms. Importantly, no plasmid was detected in the semen of patients at any time after the injections. Patients given the two highest doses of dMaxi-K had apparent sustained improvements in erectile function as indicated by improved IIEF-EF domain scores over the length of the study. One patient given 5000 (jg and one given 7500 [jg reported EF category improvements that were highly clinically significant and were also maintained through the 24 weeks of study.

Conclusions: Efficacy conclusions cannot be drawn from results of a phase 1 trial with no control group. However, the promising primary safety outcomes of the study and preliminary indications of effectiveness provide evidence that ftMaxi-K gene transfer is a viable approach to the treatment of erectile dysfunction and other smooth muscle diseases with targeted access