Phosphodiesterase Type 5 Inhibitors: a Biochemical and Clinical Correlation Survey

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International Journal of Impotence Research (2003) 15, Suppl 5, S13–S19 & 2003 Nature Publishing Group All rights reserved 0955-9930/03 $25.00 www.nature.com/ijir Phosphodiesterase type 5 inhibitors: a biochemical and clinical correlation survey

NN Kim1*

1Department of Urology, Institute for Sexual Medicine, Boston University School of Medicine, Boston, Massachusetts, USA

Phosphodiesterase type 5 (PDE 5) is the major cGMP hydrolyzing enzyme in penile corpus cavernosum and is an important regulator of nitric oxide-mediated smooth muscle relaxation. The critical role of PDE 5 in penile erection and the recent availability of specific and potent inhibitors of PDE 5 have enabled the development of effective oral treatment strategies that have been widely accepted by both health-care professionals and the lay public. This article examines the correlation between the available biochemical and clinical data for the PDE 5 inhibitors sildenafil (Viagra), tadalafil (Cialis) and vardenafil (Levitra). International Journal of Impotence Research (2003) 15, Suppl 5, S13–S19. doi:10.1038/sj.ijir.3901067

Keywords: phosphodiesterase type 5 inhibitor; sildenafil; tadalafil; vardenafil; Viagra; Cialis; Levitra; potency; efficacy; safety

Introduction differing substrate specificities based upon their ability to discriminate between cGMP and adeno- sine 30,50-cyclic monophosphate (cAMP). Further- Guanosine 30,50-cyclic monophosphate (cGMP) is an more, the activity of different phosphodiesterases important mediator of smooth muscle relaxation and may be modulated by varying mechanisms. Thus, has been shown to be a key molecular regulator of regulation of cyclic nucleotide metabolism depends penile tumescence. Intracellular cGMP concentra- on the specific expression profile of the various tions are strictly controlled by the action of guanylyl phosphodiesterase types in a given tissue. Although 1–4 cyclases and cGMP-specific phosphodiesterases. several different types of phosphodiesterases have A well-defined biochemical pathway leading to been identified within penile cavernosal tissue, penile erection involves the binding of nerve- or human trabecular smooth muscle cells express endothelium-derived nitric oxide to the heme phosphodiesterase type 5 (PDE 5) as the major moiety of soluble guanylyl cyclase within the cGMP hydrolyzing enzyme.8,9 vascular smooth muscle of the resistance arteries PDE 5 was initially identified as the major cGMP 5 and the trabeculae of the corpora cavernosa. This binding protein in bovine lung and later character- activation of guanylyl cyclase by nitric oxide ized as a cGMP-binding, cGMP-specific phospho- catalyzes the formation of cGMP from GTP. Elevated diesterase.10–12 The purified protein is a homodimer levels of intracellular cGMP lead to the activation of of 99.6 kDa subunits and binds two zinc atoms per cGMP-specific phosphodiesterases which hydrolyze monomer which are necessary for catalysis.13 PDE 5 cGMP to 50-GMP. The degradation of cGMP by has two allosteric sites for cGMP binding. Occu- phosphodiesterases is one of the main mechanisms pancy of these sites is necessary for phosphorylation by which the nitric oxide signal transduction path- of PDE 5 which has been shown to occur via protein way is terminated within smooth muscle cells. kinase G-dependent mechanisms.14,15 In experi- In all, 11 main types of phosphodiesterases (some ments using recombinant bovine PDE 5, phosphor- types with multiple isoforms) have been identified ylation of the enzyme has been shown to increase 6,7 to date. Each type of phosphodiesterase has the affinity of the allosteric binding sites for cGMP and enhance catalytic activity by 50–70%.16 Recent equilibrium radioligand binding studies indicate *Correspondence: NN Kim, PhD, Department of Urology, that sildenafil binds to the catalytic site of PDE 5 with high affinity (0.83–12 nM) and limited capacity Boston University School of Medicine, 700 Albany St. 17 W607, Boston, MA 02118, USA. (0.6 mol/mol-subunit). The presence of up to 3 E-mail: [email protected] 10 mM of cGMP enhanced the affinity of [ H]silde- PDE 5 — a survey NN Kim S14 nafil for PDE 5, most likely through interactions at the allosteric binding sites. These data suggest that sildenafil may potentiate its own action by inhibit- ing PDE 5 and causing the elevation of intracellular cGMP levels, which further increases the affinity of sildenafil for PDE 5. Whether this phenomenon is specific for sildenafil or present as a general mechanism of potentiation for any PDE 5 inhibitor remains to be seen. The critical role of PDE 5 in penile erectile physiology and the synthesis and identification of specific and potent inhibitors of PDE 5 have enabled the development of effective oral treatment strategies that have been widely accepted by both health- care professionals and the lay public. This brief review will examine the correlation between the available biochemical and clinical data for the PDE 5 inhibitors sildenafil (Viagra), vardenafil (Levitra) and tadalafil (Cialis) (see Figure 1 for chemical Figure 1 Chemical structures of sildenafil, tadalafil and varde- structures). nafil.

Potency and efficacy of PDE 5 inhibitors inversely proportional to the concentration of drug required to inhibit an enzyme (eg the lower the IC50 or Ki, the greater the potency). The potency and efficacy of any therapeutic agent is As can be seen in Figure 2, the ranking of IC50 dependent upon numerous factors. For orally ad- values was identical in two different studies utiliz- ministered PDE 5 inhibitors, these factors include: ing purified PDE 5 preparations. Vardenafil was (1) dissolution and absorption time (tmax), (2) found to be the most potent, followed by sildenafil solubility and plasma protein associations, (3) tissue and tadalafil.18,19 (E Gbekor, personal communica- and cell permeability, (4) binding and dissociation tion, 2002) Similar differences in inhibitory potency rates of inhibitors to PDE 5 (Kd,IC50 and Ki), and (5) were observed in comparing sildenafil and varde- metabolism and clearance (t1/2). Any of these factors nafil incubated with human cavernosal tissue can significantly influence clinical measures of extracts.20 These differences in potency between potency and efficacy in patients and may lead to sildenafil and vardenafil were maintained in func- inconsistencies between clinical and laboratory tional assays that measured cGMP accumulation in data. In laboratory studies, parameters of affinity cultured smooth muscle cells and erectile function and enzyme inhibitory capacity are the most in both anesthetized and conscious rabbits.20–22 common means of comparing drug potency. For With the exception of IC50 determinations, direct the purposes of this discussion, the pharmacological head-to-head comparative studies for all three PDE 5 definition of potency will be employed. Potency is inhibitors have not been reported. However, the

18 19 Figure 2 Comparison of IC50 values for PDE 5 inhibitors. See Saenz et al, Gresser and Gleiter and E Gbekor, personal communication.

International Journal of Impotence Research PDE 5 — a survey NN Kim S15 Table 1 Pharmacokinetic parameters for PDE 5 inhibitors dicated varying extents of enhancement in erectile function, all have reported significant increases in Parameter Sildenafil Tadalafil Vardenafil the percentage of successful intercourse attempts (100 mg) (20 mg) (20 mg) and the percentage of patients who responded ‘yes’ to the global assessment question (GAQ), ‘Did your t (h) 1.2 2.0 0.7 max treatment improve your erections?’. Increasingly, t1/2 (h) 3.8 17.5 3.9 Cmax (ng/ml) 327.0 378.0 20.9 studies using sildenafil have recruited patients who are considered difficult to treat for erectile dysfunc- See Corbin and Francis23 and Porst24 tion. Limited, but encouraging data indicate that Viagra significantly enhances erectile function in patients with the following conditions: diabetes mellitus, multiple sclerosis, renal failure/kidney available preclinical in vitro and in vivo data suggest dialysis, organ (heart, kidney) transplant, stable that vardenafil is a more potent inhibitor of PDE 5 cardiovascular disease with no nitrate therapy, than either sildenafil or tadalafil. Additional differ- previous use of intracavernosal injection therapy, ences between the three PDE 5 inhibitors can be SSRI therapy for depression, nerve-sparing radical seen from the pharmacokinetic data (Table 1).23,24 prostatectomy, Parkinson’s disease, spina bifida, or While the dose for sildenafil was five times higher spinal cord injury with at least partial response to than either tadalafil or vardenafil, the most notable penile vibratory stimulation.27–29 distinctions are the longer half-life (t1/2) of tadalafil While the clinical data for vardenafil and tadalafil and the lower maximal plasma concentration (Cmax) are not as extensive as sildenafil, similar outcome of vardenafil. It is likely that the prolonged half-life measures indicate that both PDE 5 inhibitors are of tadalafil results in its accumulation and leads to a effective in treating men diagnosed with erectile higher plasma concentration than vardenafil dysfunction.24,30–34 Separate studies indicate that although equal doses of each inhibitor were admi- vardenafil significantly improves erectile function nistered. in men with type I or type II diabetes mellitus and The time of onset of action for each PDE 5 also in men with a history of radical prostatect- inhibitor is not readily available. However, in a omy.35 In a recently published phase 3, randomized, small study of 17 patients administered 50 mg of double-blind, placebo-controlled, multicenter study sildenafil after visual sexual stimulation, the med- performed at sites in the US and Canada, vardenafil ian onset time to achieve penile erection with significantly increased the percentage of successful greater than 60% rigidity (assessed by RigiScan) intercourse attempts by 50–100%, depending on was 27 min after dosing.25 In studies using success- dose.32 Similar degrees of enhancement were re- ful intercourse completion as the primary measure ported for the percentage of patients responding of efficacy, 20 mg of tadalafil was observed to be positively to the GAQ. In addition, the effectiveness effective as early as 16 min after dosing in 32% of of vardenafil at the midpoint of the study (12 weeks) men (vs 15% for placebo).26 Although no compar- persisted through the end of the treatment period able published data are available for vardenafil in (26 weeks). humans, it is reasonable to assume from the tmax that An important caveat in interpreting the data from a significant proportion of men taking vardenafil this vardenafil study is that patients with a history would experience enhanced erectile function within of failing sildenafil therapy were specifically ex- 30 min. The efficacy of these compounds does not cluded. In the group of patients who experienced appear to be significantly affected by food or alcohol improved erections with vardenafil, 71% reported intake, since most clinical trials did not restrict previously using sildenafil successfully. The exclu- these parameters. sion of sildenafil nonresponders in this study may By virtue of its earlier availability, the greatest increase the percentage of patients responding amount of clinical data has been accumulated for positively to vardenafil. On the other hand, it is sildenafil. It is estimated that more than 13 million likely that the patients enrolled into the study, on patients have been treated with Viagra worldwide.27 average, had less severe erectile dysfunction. This In a meta-analysis of 27 clinical trials (randomized, would cause elevated placebo or baseline values and placebo-controlled studies lasting at least 7 days) potentially lead to smaller degrees of enhancement involving 6659 men, Fink et al28 reported that with drug administration (Table 2). This may be the sildenafil administration resulted in a significant reason for the reported 50–100% enhancement in increase in the mean score for question #4 on the successful intercourse attempts for vardenafil vs the International Index of Erectile Function (IIEF) and 150–250% increase reported for sildenafil. It should in the percentage of successful intercourse attempts be noted that similar exclusion criteria were used for per patient. For the 25 mg dose, a 1.5-fold increase the clinical assessment of tadalafil33 and published was observed over placebo, whereas a 2.5-fold data indicate similar trends (ie higher scores for increase over placebo was observed for the 50 and placebo and active drug with smaller relative 100 mg doses. While individual studies have in- increases when compared to sildenafil).

International Journal of Impotence Research PDE 5 — a survey NN Kim S16 Table 2 Comparison of assessment scores (percent of successful intercourse attempts per patient) for erectile function in studies using sildenafil, vardenafil or tadalafil. For each study, data for the maximal effective drug concentration are shown

Placebo Active % Increase Reference drug

Sildenafil (100 mg) 14 51 264 26 Vardenafil (20 mg) 32.7 66.7 104 30 Tadalafil (25 mg) 26.6 70.2 164 31,32

A more recent clinical evaluation of tadalafil enrolled 1112 men with a broad spectrum of severity and etiology for erectile dysfunction.34 The study population had a mean age of 59 y and included men with hypertension, coronary artery disease, diabetes mellitus and depression. Tadalafil significantly increased most of the efficacy outcomes in a dose- dependent manner. A unique aspect of the study examined the percentage of successful intercourse attempts over time after drug administration. Effi- cacy was maintained up to 36 h after dosing without diminishment, correlating with the longer half-life of tadalafil. Given that vardenafil has been shown to enhance erectile function after four half-lives in a conscious rabbit model,36 it is likely that tadalafil would remain efficacious well beyond the measured 36 h time point. Although direct head-to-head comparative clinical data from the same study are not currently available, data from studies using a single PDE 5 inhibitor suggest that differences in biochemical potency may be carried forward to the clinical setting (Figures 3 and 4). This notion is verified by the range of dosages used clinically and the effective Figure 3 Clinical efficacy of PDE 5 inhibitors as assessed by successful intercourse attempts per patient. Asterisk indicates plasma concentrations achieved by each PDE 5 significant increase over baseline. See Porst et al,26,31 Hatzichris- inhibitor. However, it should be noted that at the tou,27 Goldstein et al,29 Pryor,30 and Hellstrom et al.32 normally prescribed doses, the efficacy of all three PDE 5 inhibitors is similar. As discussed previously, the time window during which tadalafil remains efficacious is probably greater due to its prolonged half-life and the resulting elevation in plasma drug concentration.

Safety and tolerability of PDE 5 inhibitors

The safety and tolerability of sildenafil, vardenafil and tadalafil are dependent upon the tissue-specific distribution of phosphodiesterases, the selectivity or inherent cross-reactivity of each drug with phosphodiesterase subtypes (other than PDE 5) and Figure 4 Clinical efficacy of PDE 5 inhibitors as assessed by biological action unrelated to phosphodiesterase global assessment question (GAQ). See Porst et al,26,31 Hatzi- inhibition. In general, sildenafil, vardenafil and christou,27 Goldstein et al,29 Pryor,30 and Hellstrom et al.32 tadalafil all have excellent selectivity for PDE 5. However, sildenafil is somewhat less selective selective for PDE 6, whereas tadalafil is least against PDE 1 and PDE 6 (Table 3).18,19 (E Gbekor, selective for PDE 11. These varying selectivity personal communication, 2002). Vardenafil is less profiles may influence adverse event outcomes as

International Journal of Impotence Research PDE 5 — a survey NN Kim S17 Table 3 Comparison of selectivity ratios for sildenafil, vardenafil and tadalafil

Inhibitor PDE 1 PDE 6 PDE 11

Sildenafil 60–80 7–20 200–800 Vardenafil 250–500 4–16 400–1200 Tadalafil >4000 200–800 5–7

The selectivity ratio of a given inhibitor is defined as the IC50 value for a given PDE subtype divided by the IC50 value for PDE 5. (See Saenz et al,18 Gresser and Gleiter19 and E Gbekor, personal communication, 2002) ever larger numbers of patients are administered PDE 5 inhibitors. PDE 1 is mainly located in vascular smooth muscle, heart and brain, whereas PDE 6 is strictly localized in the retina. PDE 11 has a wide tissue distribution, including skeletal muscle, testes, prostate, breast and corpus cavernosum.37,38 While the gene for PDE 11 has been cloned and two different splice variants have been identified,38,39 little is known about its biological role(s) and the consequences of PDE 11 inhibition in the various tissues are difficult to predict. Adverse event profiles arising out of clinical trials have provided the most valuable information, thus far. The currently available clinical data suggest that none of the PDE 5 inhibitors cause serious adverse events. General assessments of cardiovascular function such as blood pressure measurements and electrocardiograms indicate that there are no clinically relevant adverse changes due to administration of sildenafil, vardenafil of tadalafil.32,33,40 The most common adverse events for men taking PDE 5 inhibitors have been headache, cutaneous flushing, nasal congestion and dyspepsia. The occurrence of these adverse events was usually dose-dependent and mild to moderate in severity. The prevalence of three commonly reported adverse events in PDE 5 inhibitor studies is shown in Figure 5. For comparative purposes, data for a single common dose (or the closest alternative) are given for each PDE 5 inhibitor. In addition to these common adverse events, approximately 1–11% of study participants taking varying doses of sildenafil reported visual Figure 5 Adverse events for clinical studies with sildenafil, vardenafil and tadalafil. See Porst et al,26,31 Hatzichristou,27 disturbances such as changes in acuity, color or 29 30 32 Goldstein et al, Pryor, and Hellstrom et al. brightness.40 For vardenafil, less than 2% of study participants reported visual changes and none of these indicated blue color vision.32 It is unclear if An additional concern regarding inhibition of any of the visual changes were treatment related, PDE 11 has been that of testicular function. In a since other vardenafil studies report no abnormal recent randomized, double-blind, placebo-con- visual disturbances. Similarly, no significant trolled study, the effects of tadalafil on sperm changes in color vision were reported by tadalafil characteristics were examined.41 Tadalafil at the study participants. Unique to tadalafil treatment 20 mg dose was administered daily for 6 months to are complaints of back pain, which tend to increase men who were healthy or had mild erectile slightly at higher doses (4–9% for drug vs 0–5% dysfunction. Parameters of sperm morphology, for placebo).25,33,34 The cause of the back pain motility, number and concentration were measured remains unclear, although PDE 11 is suspected to along with serum testosterone, luteinizing hormone play a role. and follicle stimulation hormone. Tadalafil was

International Journal of Impotence Research PDE 5 — a survey NN Kim S18 found to have no significant effect on hormone 9 Stacey P, Rulten S, Dapling A, Phillips SC. Molecular cloning levels or on the characteristics of sperm with the and expression of human cGMP-binding cGMP-specific exception of attenuating a decrease in sperm phosphodiesterase (PDE5). Biochem Biophys Res Commun 1998; 247: 249 – 254. motility. These data suggest that continual inhibi- 10 Francis SH, Lincoln TM, Corbin JD. Characterization of a tion of PDE 11 has no detrimental effects on novel cGMP binding protein from rat lung. J Biol Chem 1980; spermatogenesis or hormone levels. However, 255: 620 – 626. further studies are warranted to confirm and extend 11 Thomas MK, Francis SH, Corbin JD. Characterization of a purified bovine lung cGMP-binding, c-GMP-specific phospho- these findings. diesterase. J Biol Chem 1990; 265: 14964 – 14970. Owing to the potential hypotensive action of PDE 12 McAllister-Lucas LM et al. The structure of a bovine lung 5 inhibitors, all concomitant nitrate use is contra- cGMP-binding site of cGMP-binding, c-GMP-specific phos- indicated. Potential interactions also exist with phodiesterase deduced from a cDNA clone. J Biol Chem 1993; alpha-adrenergic blockers. In addition, since these 268: 22863 – 22873. 13 Francis SH, Colbran JL, McAllister-Lucas LM, Corbin JD. Zinc PDE 5 inhibitors are predominantly degraded by the interactions and conserved motifs of the cGMP-binding, cytochrome P450 family of hepatic microsomal c-GMP-specific phosphodiesterase suggest that it is a zinc enzymes, use of drugs that significantly compromise hydrolase. J Biol Chem 1994; 269: 22477 – 22480. this catabolic system (eg erythromycin, ketocona- 14 Turko IV, Francis SH, Corbin JD. Binding of cGMP to both allosteric sites of cGMP-binding cGMP-specific phosphodies- zole, itraconazole, cimetidine, saquinavir) may terase (PDE5) is required for its phosphorylation. 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