(PDE11): Is It a Player in Human Testicular Function?

Phosphodiesterase 11 (PDE11): is it a player in human testicular function?

SH Francis1*

1Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennesse, USA

Editors Note: PDE11, the newest member of the PDE family of phosphodiesterases, has become the center of controversy. Four splice variants were recently identified, PDE11A1–4. Historical data have suggested that PDE11A3 is found in the testis while PDE11A4 is found in the prostate. The controversial issue is the inhibition of PDE11 by tadalafil. In the light of tadalafil’s commercial success, its inhibition of PDE11A has been the subject of heated debate. A variety of published reports addressed this issue, suggesting that the target organ of tadalafil’s inhibition, the testis, is not adversely affected. Daily tadalafil given to healthy volunteers did not alter semen analysis parameters or blood hormonal parameters, seemingly mitigating the clinical effect of the PDE11 inhibition. However, two recent papers published in this journal have added fuel to this proverbial fire. In this perspective, Sharron Francis, a noted PDE expert, and a co-author of one of the recent papers mentioned above, sheds further light on this contested topic. International Journal of Impotence Research (2005) 17, 467–468. doi:10.1038/sj.ijir.3901377; published online 4 August 2005

This Perspective discusses two recent reports in tion and further address the safety profile of IJIR; both focused on phosphodiesterase-11 (PDE11), tadalafil. which hydrolyzes the cyclic phosphate ring in There is strong interest in PDE11 since its cAMP and cGMP, thereby inactivating these mole- physiological role is unknown, and its structure is cules as second messengers. The reports used most closely related to phosphodiesterase-5 (PDE5), different approaches to study the physiological role the target of commercially marketed inhibitors, of PDE11 and implications for potential tadalafil sildenafil (Viagras), vardenafil (Levitras), and tada- (marketed by Lilly ICOS as Cialiss) inhibition of lafil (Cialiss). Despite having 50% identity in amino- PDE11 in men using this medication for treatment acid sequence with the catalytic domain of PDE5, of erectile dysfunction (ED). Loughney, Taylor and PDE11 is not significantly inhibited by sildenafil or Florio rigorously assessed distribution of PDE11 in vardenafil, but tadalafil has modest inhibitory human tissues, an important contribution since potency. Tadalafil reportedly inhibits catalytic activ- there are conflicting reports on this topic in the ity of one PDE11 isoform (PDE11A1) at concentra- literature. Wayman et al used PDE11 knockout tions within the therapeutic range (Cialiss package mice to examine a role of this enzyme in sperm insert, Lilly ICOS, 2003). Nevertheless, the like- physiology and as a basis for considering potential lihood that tadalafil significantly inhibits PDE11 implications of inhibition of PDE11 activity by activity has been questioned based on the B40-fold tadalafil. In combination with results from earlier weaker potency for PDE11A4 than for PDE5. How- studies, these reports have generated considerable ever, tadalafil market share for treatment of ED is discussion in the scientific community and will growing rapidly, and some individuals are opting for hopefully encourage more studies of PDE11 func- regularly-timed administration of the medication rather than a more sporadic ‘on-demand’ usage. The paucity of information regarding PDE11 function combined with the potential for tadalafil inhibition of its activity is a source of some concern *Correspondence: SH Francis, PhD, Department of Mole- cular Physiology and Biophysics, Vanderbilt University in the medical and scientific communities. School of Medicine, 21st and Garland, Nashville, TN The Loughney studies were very straight- 37232-0615, USA. forward, that is, if PDE11 is important in a tissue, E-mail: [email protected] one would expect to find the protein in some Received 5 June 2005; accepted 5 June 2005 abundance therein. In human tissues, PDE11A4 Phosphodiesterase 11 (PDE11): a player in human testicular function? SH Francis 468 was the only isoform identified and was detected This could be due either to no role for PDE11 in the only in the liver, pituitary, glandular epithelium of testis or compensatory upregulation of other PDEs prostate, and neuronal cells in parasympathethic that can replace the function normally provided by ganglia in the heart. PDE11 was not found in the PDE11. testis, penis, or skeletal muscle. The merit of these Interpretation of results derived from studies of results depends entirely on the specificity and the knockout animals has significant limitations and sensitivity of the methodology used. One can never caution is indicated. A major consideration is the be absolutely positive that experiments are flawless, well-established species variation in tissue expres- but by all reasonable measures, the approaches used sion of proteins; in many instances, nature has used by Loughney and colleagues were rigorous, highly different complements of closely related proteins to sensitive, and well controlled. First, they used a achieve the same end effect in a given tissue. The collection of human tissues, and the testicular tissue Loughney studies could not detect PDE11 in samples were derived from three individuals and a the human testis. While the presence of PDE11 pool of tissue from 18 men (age range 19–64 y). in the human testis cannot be entirely excluded, Second, PDE11 protein standards for all four iso- these results argue against a major role for PDE11. forms were employed to verify appropriate molecu- This is in contrast to the apparent presence of lar weights for proteins detected by the antisera. PDE11 in murine testis and sperm. Third, two potent antibodies specific to different While PDE11 may be abundantly expressed and regions of the catalytic domain shared by all PDE11 have a modest role in murine sperm physiology, isoforms were used; this, in combination with implications for human sperm function are ques- comparison of the migration of the bands with tionable. Other workers have questioned the PDE11 protein standards, lends veracity to identifi- relevance of the rat or dog as models for studying cation of the protein bands. Fourth, the work the effect on spermatogenesis. In addition, the focused on PDE11 proteins and not on mRNA importance of a role for PDE11 in murine sperm transcripts; despite a common misconception that physiology is questionable since its absence did not mRNA levels mirror those of the protein product, impair fertility or viability of offspring. PDE11-null myriad examples demonstrate this to be untrue. mice provide a model of an animal with chronically In these studies, Loughney and co-workers were and quantitatively inhibited PDE11. In these mice, unable to detect PDE11 in the human testis. This PDE11 is absent throughout development and does not exclude the possibility that PDE11 could be adulthood; currently, only adult male patients are present in either very low levels or in minor exposed to tadalafil and the potential subcellular compartments, but it weakens the case for inhibition of PDE11 is typically limited. Even for a major role of PDE11 in the human testis. though tadalafil may reach plasma concentrations Results from this study should also focus more capable of partially blocking PDE11 activity, this is attention on potential effects of PDE11 inhibition in most likely a transient phenomenon, and based on pituitary, liver, prostate glandular epithelium, and dosing regimens followed by most individuals, few neurons in the heart. men are exposed to such levels on a daily basis. Wayman et al took a very different approach by Therefore, a persistent and high degree of PDE11 creating a PDE11 knockout mouse in an effort to inhibition seems unlikely. uncover a physiological role for PDE11. Their report Given the concern for potential tadalafil inhibi- documented PDE11 expression in the testis and tion of PDE11 within therapeutic range and the developing spermatogonia of wild-type mice. Mod- dearth of information regarding the physiological est changes were noted in ejaculated sperm from role of PDE11 in humans, it seems reasonable to PDE11-null mice including reduction in forward suggest more comprehensive clinical studies parti- progression of sperm (30%) with no change in cularly relating to the effect of tadalafil on testicular overall motility, sperm concentration (20%), and function, germ cell viability, and characteristics of live sperm (20%). Pre-ejaculated sperm of PDE11- prostatic fluid. The most comprehensive study null mice were somewhat more likely (29%) to have included only men above 45 y of age who suffer undergone capacitance compared to wild-type mice from mild ED; this study found no adverse effects of (19%). Despite this, the fertility rate of PDE11-null daily intake of tadalafil, but some criticisms have mice was comparable to that of wild-type mice, and been voiced. The complex biology of cyclic nucleo- there were no apparent differences in viability, tide signaling and modulation of these pathways histological features, or anatomical abnormalities. by medications such as tadalafil provide impetus These results suggest that PDE11 may have some for continued efforts to develop a thorough under- function in murine sperm, but its absence does not standing of the effects of these compounds and translate into a functionally significant phenotype. validation of their safety profiles.

International Journal of Impotence Research