Sildenafil, a cGMP-specific phosphodiesterase type 5 inhibitor, showed no beneficial effects in preventing anthracycline-induced CTX during a phase I/II clinical trial, despite it being approved for the treatment of erectile dysfunction and pulmonary hypertension, due to its vasodilatory effect mediated by cGMP/PKG upregulation (129). The gene discussed is PRKG1; the disease is pulmonary arterial hypertension.