(3 trial with
`l Urol 1966;
`
`Aarcolin G.
`nous bodies
`moxisylyte
`as 1991; 3:
`
`Priapismen.
`A 1939: 28:
`
`'acavernous
`ts: a dose
`-305.
`
`ition: prep-
`l Impotence
`
`31in G. Les
`E1: an real
`tions intral
`
`)staglandin
`motion. Int
`
`)apaverine~
`in intraca-
`:tion. Int
`I
`
`ion: effica-
`intracaver~
`P1521.
`glandin E1.
`nter study
`Res 1994;
`
`LCHVEInOuS
`ale erectile
`Jrol 1936 ;
`
`l, Mensinlc
`:hological,
`tence Res
`
`7K
`
`
`
`International Journal of Impotence Research (1996) 8, 47-52
`
`095579930/96All rlghts reserved© 1996 Stockton Press. $12.00
`
`
`
`'~»,__,.
`
`to
`
`Sildenafil: an orally active type 5 cyclic GMP-specific
`phosphodiesterase inhibitor for the treatment of penile erectile
`dysfunction
`
`Mitradev Boolelll, Michael I Allenl, Stephen A Ballard], Sam Gepi—Atteez, Gary I Muirhead',
`Alasdair M Naylorl, Ian H Osterlohl and Clive Gingell2
`
`1 Pfizer Central Research, Sandwich, Kent CT13 9N], UK; 2 Urology Department, Southmead Hospital, Bristol, UK
`
`Sildenafil [Viagram, UK-92,480] is a novel oral agent under development for the treatment of
`penile erectile dysfunction. Erection is dependent on nitric oxide and its second messenger.
`cyclic guanosine monophosphate [cGMP]. However, the relative importance of phosphodiester-
`ase [PDE]
`isozymes is not clear. We have identified both cGMP- and cyclic adenosine
`monophosphate-specific phosphodiesterases (PDEsl in human corpora cavernosa in vitro. The
`main PDE activity in this tissue was due to PDE5, with PDEZ and 3 also identified. Sildenafil
`is a selective inhibitor of PDE5 with a mean IC50 of 0.0039pM. In human volunteers, we have
`shown sildenafil to have suitable pharmacokinetic and pharmacodynamic properties [rapid
`absorption, relatively short half-life, no significant effect on heart rate and blood pressure} for
`an oral agent to be taken, as required, prior to sexual activity. Moreover, in a clinical study of
`12 patients with erectile dysfunction without an established organic cause, we have shown
`sildenafil to enhance the erectile response (duration and rigidity of erection] to visual sexual
`stimulation, thus highlighting the important role of PDES in human penile erection. Sildenafil
`holds promise as a new effective oral treatment for penile erectile dysfunction.
`
`Keywords: penile erectile dysfunction; sildenafil; oral treatment; phosphodiesterase type 5
`
`Introduction
`
`Penile erectile dysfunction is a common medical
`disorder. It has an estimated prevalence of 2% in
`men aged 40 years, which increases to over 50%
`in men over the age of 70 years.1 Penile erectile
`dysfunction has been defined as ‘the inability to
`achieve and/or sustain an erection for satisfactory
`sexual performance‘.2 Generally,
`it
`is accepted
`that this disorder adversely affects quality of life.
`Patients often report
`increasing anxiety,
`loss of
`self-esteem,
`lack of self—confidence,
`tension and
`difficulty in the relationship with their partner.2
`Penile erection is a haemodynamic event which
`is dependent upon relaxation of
`the smooth
`muscle cells of the corpus cavernosum and of its
`associated arterioles, with consequential increase
`in arterial flow into the trabecular spaces of the
`corpora cavernosad“; The increased blood flow
`causes the lacunar spaces or sinusoids to become
`distended which results in compression of the
`small venules between the sinusoids and the
`
`tunica albuginea. The relative indistensibility of
`the tunica albuginea results in a vane-occlusive
`effect such that the penile pressure increases to
`
`Correspondence Dr M Boolell
`Received 25 April; accepted 15 May 1996
`
`ATI 1023-0001
`
`approach mean arterial pressure and penile rigid-
`ity develops.
`There is now ample evidence from both animal
`experiments and in vitro studies with human
`tissue to suggest
`that relaxation of the smooth
`muscle of the corpora cavernosa is mediated by
`nitric oxide via cyclic guanosine monophosphate
`[cGMP].5’8 During sexual stimulation, nitric oxide
`is released from nerve endings and endothelial
`cells. Nitric oxide then stimulates the cytosolic
`enzyme guanylate cyclase to produce CGMP which
`results in a decrease in intracellular calcium and
`
`allows relaxation of smooth muscle cells. Cyclic
`nucleotide phosphodiesterase
`(PDE)
`isozymes,
`which are distributed in various tissues, specifi-
`cally hydrolyse cyclic nucleotides, such as cGMP.9
`Therefore, a pharmacological agent which inhibits
`the
`cGMP-specific phosphodiesterase
`isozyme,
`should enhance the action of nitric oxide/cGMP on
`penile erectile activity and have the potential to
`enhance penile erections during sexual stimu-
`lation.
`
`therapy for penile
`To date, pharmacological
`erectile dysfunction has been largely based on the
`use of
`intracavernosal
`injections of vasoactive
`agents. Though efficacious, this form of therapy is
`associated with a high dropout rate for a variety
`of reasons.10 Recent insights into the mechanism
`of penile erection have, however, led to the devel—
`
`ATI V. ICOS
`
`———-—IP-R29—t8a91 183
`
`ATI 1023-0001
`
`ATI v. ICOS
`IPR2018-01183
`
`

`

`Sildenafi l
`M Boolell eta/
`
`[
`.
`TM
`; 1- 4-ethoxy-3-(6,7-
`opment of sildenafil (Vragr_a
`dihydro-1-methyl-7-oxo-3-piopyl-1H-pyrazolo [ 4,3 -
`d]
`pyrimidin-5-yl)
`phenylsulphonyl]-4-methyl(cid:173)
`piperazine, pfizer Central Research) a nov~ l,
`or ally-efficacious drug for the treatment of p emle
`erectile dysfunction.
`.
`.
`.
`.
`This review focuses on lll v1tro studies relatmg
`to
`the mode of action of sildenafil, phar(cid:173)
`macokinetic s tudies in human vo lunteers , and an
`early clinical study in patients with erectile dys-
`function.
`
`Methods
`
`Iso lulion of soluble ph os ph odie s te ru se act ivities from
`human ti ss ues
`
`Frozen human corpus cavernosal tissue (obtained
`from IIAM, Exton, Pennsylvania; donor age r ange
`48- 64 years) was thawed on ice, coarsely chopped,
`and
`then homogenised
`in approximately 4
`volumes of ice cold HEPES buffer (20 mM, contain(cid:173)
`ing 0.25 M sucrose, 1 mM EDT A, 1 mM phenyl(cid:173)
`methyl sulphonylfluoride [PMSF], pH 7.2) using
`an Ultra Turrax homogeniser at high setting. The
`homogenate was filt ered through two layers of
`surgical gauze to remove any undisp er sed tissue
`and fibrous material. The filtrate was centrifuged
`at 100 000 g for 60 min at 4°C. The s upernatant
`was filtered through a 0.2 ~LM filter and either
`u sed directly (see below) or s tored in liquid nitro (cid:173)
`gen prior to analysis . A similar procedure was
`used for preparation of soluble fractions from
`samples of human cardiac ventricle and human
`skeletal muscle (also obtained from IIAM).
`Phosphodiesterase activities in the soluble frac (cid:173)
`tions prepared from human tissues were separat(cid:173)
`ed using a Pharmacia FPLC system (Pharmacia
`Ltd, Milton Keynes, UK) with a Mono Q anion
`exchange column (1 mL bed volume, Pharmacia
`Ltd). The Mono Q co lumn was pre-equilibrated
`with HEPES buffer
`(20 mM, containing 1 mM
`EDTA, 0.5mM PMSF, pH 7.2), before loading
`tissue soluble fractions (2- 5 mL). The column was
`then washed with 5 ml of the buffer and the phos (cid:173)
`phodies terase isozymes were eluted using a con-
`
`tinuous gradient of 0-500 mM NaCl in the same
`buffer (tota l volume 55 mL) at a flow rate of 1 mL/
`min, and 2 mL fra ctions wer e collected. Fractions
`comprising the main peaks of pho sp hodies tera se
`activity were pooled and stored at - 80°C for use
`in characterisation a nd inhibition studies.
`
`Dete rmination oJ phos ph odies terase activity and
`isozym e characteri sa tion
`The cyclic nucleotide phosphodiesterase activity
`in FPLC fraction s was determined u sing a modifi(cid:173)
`cation of the two step radioisotopic procedure of
`Thomp son and Appleman. 11 The reaction mixture
`(total volume 100 ~tL) contained co lumn fraction
`(10 to 25 ~tL) , [ 3H]-cGMP or [ 3H}cAMP (500 nM,
`2 ~tCi/m L), bovine serum albumin (0.5 mg/ mL) and
`MgC1 2 (5 mM) in Tris HCl buffer (15 mM, pH 7.4).
`Reaction was initiated by addition of the ra diola(cid:173)
`belled substrate, and the samp les incubated in a
`water bath at 30"C for 30 min. The reaction was
`stopped by immersing the samp le tubes in boiling
`water for 2 min. The 5'-mo nonucletides formed by
`hydrolysis of cyclic nucleotides were determined
`after their further conversion to nucleo sides using
`s nake venom (Oph iophagus hannah) nucleotidase
`activity as described by Thomp son and Apple(cid:173)
`man.1 1
`test compounds were
`inhibitor studies,
`For
`added to incubation mixtures in dimethyl sul(cid:173)
`phox ide (DMSO, final concentration 2°/cl v/v). The
`hydrolysis of cyclic nucleotides did not exceed
`15 1YcJ and under these conditions, product forma (cid:173)
`tion increased linearly with time and amount of
`enzyme .
`from
`IC 50 values were determined
`sigmoidal curves, fitted to plots of enzyme activity
`vs
`log compound concentration using a curve
`fitting programme. 1 2
`Pho sphodies terase activities in FPLC fraction s
`were characterised based on their substrate spe(cid:173)
`cificities, effects of calcium calmodulin, the effect
`of cGMP on cAMP hydrolytic activity and the
`inhibitory potency of known selective inhibitors of
`the PDE isozymes (Tab le 1). 13
`16 Human cardiac
`-
`ventricle was used as a source of soluble PDE1
`and human s keletal muscle (gluteus m aximus) for
`PDE4 .
`
`Table 1 Classification of PDE isozymes 1 3
`
`Isozyme
`.fi11nily
`
`PDE I
`PDE 2
`PDE 3
`PDE 4
`PO E 5
`
`Suhstrat e
`specificity
`
`cA MP/ cGMP
`cAMP/ cGMP
`cAMP
`cAMP
`cG MP
`
`Effect of cG M P un
`cA MP hydrolysis
`
`Effect uj calcium
`ctllmorlulill
`
`N.A.
`s timulation
`inhibition
`no effect
`N.A.
`
`stimulation
`no effect
`no efi"ect
`no effect
`no efi.e ct
`
`Slamlarrl inhihil or
`
`vin pocetine 1 h
`none
`m ilrinone 16
`ro li pram 1
`''
`
`zaprinast 1 '', E4021 15
`
`N.A. - not ap propri ate for chara c terisation of these isozyme families
`PDE = phos phodi es ter ases
`cGMP =cyclic g uano sine rnonopho s ph a te
`cAMP = cyclic adenosin e monopho sphate
`
`ATI 1023-0002
`
`

`

`Pharmacokinetic studies
`The following studies have been conducted to
`investigate the single oral dose pharmacokinetics
`ofsildenafil.
`(i) A single-blind, escalating single oral dose
`study, in which solution doses ranging from
`to 90 mg were administered
`to
`two
`1 .25
`groups (n = 9 and n = 10) of healthy male
`volunteers, with random insertion of placebo.
`Each subject received three active doses and
`placebo.
`(ii) An extension of the first study in which a
`group of 10 healthy ~ale volunteers received
`single oral solution doses of sildenafil (100,
`150 and 200 mg) with random insertion of
`placebo.
`two-way cross-over
`(iii) An open, randomised,
`study to investigate the pharmacokinetics of
`sildenafil after oral administration
`(50 mg
`capsule)
`and
`intravenous
`administration
`(50 mg) to a group of 12 healthy male volun(cid:173)
`teers .
`In all studies blood samples were taken pre-dose
`and at intervals up to 72 h post-dose for determi(cid:173)
`nation of the plasma concentrations of sildenafil.
`Plasma and urine samples were assayed using a
`sensitive and specific HPLC assay following solid
`phase extraction.
`
`Clinical study
`The efficacy of sildenafil on penile erectile activity
`was evaluated in 12 patients who had a history of
`penile erectile dysfunction of at least six months
`duration. On clinical evaluation there was no
`obvious organic cause for penile erectile dys(cid:173)
`function. Patients were excluded from the study if
`there was evidence of neurovascular disease on
`clinical evaluation, diabetes, drug or alcohol
`abuse, or other estab lished causes for their penile
`erectile dysfunction.
`double-blind, placebo(cid:173)
`a
`The
`study was
`controlled, randomised, four-way crossover design.
`A period of at leas t
`three days was allowed
`between consecutive treatment periods to ensure
`that there was adequate clearance of sildenafil
`from
`the circulation. On each dosing period,
`patients were admitted to a hospital bed with
`complete privacy. They received a single dose of
`sildenafil (10, 25 or 50 mg) or placebo. Each dose
`was followed by visual sexual stimulation (VSS)
`starting 30 min post-dose and la sting for
`two
`hours . VSS was provided by viewing of sexually
`explicit material chosen from a selection of videos
`and magazines. Drug efficacy on penile erectile
`activity was evaluated by measurement of penile
`rigidity at the base and tip of the penis by penile
`plethysmography (RigiScan, Dacomed Corpora(cid:173)
`ion). The study was approved by the local ethics
`committee.
`
`/
`
`/
`
`49
`
`Sildenafil
`M Boolell et a/
`
`Mean duration of rigidity of greater than 60% at
`the base and tip of the penis was analysed by
`analysis of variance (ANOV A).
`
`Results
`
`Phosphod iesterase activities in human corpus
`covernosum
`
`Two major peaks demonstrating cGMP hydrolytic
`activity and one peak demonstrating cAMP hydro(cid:173)
`lytic activity were resolved by anion exchange
`chromatographic analysis of soluble
`fractions
`from human corpus cavernosum (Figure 1). Peak 1,
`which had the greatest PDE activity, was found in
`soluble fractions prepared from three separate
`samples of human corpus cavernosum. The PDE
`activity in the peak was specific for cGMP as sub(cid:173)
`strate and unaffected by calcium calmodulin. In
`
`(a)
`
`100
`
`-o- 0.5 ~M cGMP
`--~:>-- 0.5 ~M cGMP + 100 nM sildenafil
`
`so
`
`E
`c
`:§
`~ 60
`Eo
`c
`·:;:
`t5
`<tl
`w
`12
`
`40
`
`20
`
`14
`
`16
`
`18
`
`24
`22
`20
`Fraction number
`
`26
`
`28
`
`30
`
`-o- 0.5 ~M cAMP
`--~:>-- 0.5 ~M cAMP + 100 nM sildenafil
`
`Peak 3
`(PDE3)
`
`/
`
`(b)
`
`100
`
`E
`c
`:§
`0
`E
`Eo
`c ·:;:
`~
`<tl
`w
`0
`a..
`
`80
`
`60
`
`40
`
`20
`
`0
`
`26
`
`28
`
`14
`
`16
`
`18
`
`24
`22
`Fraction number
`Figure 1 Separation of POE isoenzymes from human corpus
`cavernosum by fast protein liquid chromatography on a
`Mono-Q co lumn . Panel (a) shows hydrolysis of cGMP (0.5 pM)
`in the presence and absence of 100 nm sildenafil. Panel (b)
`shows hydrolysis of cAMP (0.5 pM)
`in the presence and
`absence of 100 nm sildenafil. Each point represents the mean
`of three replicate assays
`
`ATI 1023-0003
`
`

`

`Sildenafil
`M Boolell et a/
`
`addition it was inhibited by zaprinast, a s tandard
`inhibitor with a mean IC 5 0 ± SEM of
`PDE5
`0.86 ± 0.11 ~ (n = 4), which is similar to values
`previousl1 reported
`for PDE5
`from rat
`lung
`(0.76 ~) 7 and porcine aorta (0.51 J..lM). 15 These
`data indicated that peak 1 comprised predomi(cid:173)
`nantly PDE5. Peak 2 and peak 3 were identified as
`PDE2 and PDE3, respectively.
`
`Effects of sildenafil on human phosphodies teras e
`activities
`Sildenafil (100 nM) completely inhibited the cGMP
`hydrolytic activity of the PDE5 peak from human
`corpus cavernosum, but had no significant effect
`of the PDE2 or PDE3 peaks (Figure 1). Moreover,
`sildenafil was found to be a potent inhibitor of
`PDE5 activity from human corpus cavernosum
`with a mean IC 50 of 0.0039 f.!M (Table 2). Sildenafil
`had low activity against PDE2 and PDE3 from
`corpus cavernosum and PDE4
`from skeletal
`muscle (IC 5 0 values > 7.3 J..lM; Table 2). It had
`moderate activity against PDE1
`from human
`cardiac ventricle
`(IC 5 0 , 0.29 f.!M) . These data
`showed that sildenafil was at lest 70-fold selective
`for PDE5 relative to isozymes from PDE families
`1- 4.
`
`Pharmacokinetics of sildenafil in human
`volunteers
`The results from the single-blind escalating single
`oral dose
`(studies
`[i] and
`[ ii] under phar(cid:173)
`macokinetic method) demonstrate that sildenafil is
`rapidly absorbed with maximum observed plasma
`concentrations occurring within one hour after
`oral dosing. Plasma concentrations decline in a hi(cid:173)
`exponential manner with a mean terminal half life
`of 3 to 5 h. Pharmacokinetic simulations predict no
`significant accumulation of the drug after repeat(cid:173)
`ed once-daily dosing.
`sildenafil were
`of
`The
`pharmacokinetics
`approximately dose proportional after adminis(cid:173)
`tration of solution formulations
`in
`the range
`1.25 mg to 200 mg. The maximum plasma concen(cid:173)
`tration
`and
`the
`area under
`the
`plasma
`concentration-time curve increased linearly with
`dose. (Figures 2 and 3).
`The absolute bioavailability (study[iii]above)
`revealed that sildenafil has a mean plasma clear(cid:173)
`ance of 41 L/ h and a mean steady state volume of
`
`:::;-
`E
`0,
`.s
`~
`E
`0
`
`2500
`
`2250
`
`2000
`
`1750
`
`1500
`
`1250
`
`1000
`
`750
`
`500
`
`250
`
`0
`
`0
`
`y = 7.53x- 47.7
`
`r2 = 0.99
`
`25
`
`50
`
`75
`
`100 125 150 175 200
`Dose (mg)
`
`pl ~sma concentr~t~on ~alues
`Figure 2 Mean ma x imum
`) of sildenafil followin g sm gle oral admm1s trat10n to
`(C
`he,~lthy male volunteers. Each point represents mean ± SD
`from 9 or 10 subjects
`
`distribution of 105 L. The mean absolute bio(cid:173)
`availability after oral dosing of a 50 mg capsule
`was 41%.
`There were no clinically significant effects on
`pulse rate, blood pres sure and laboratory safety
`tests (haematology and biochemistry profiles) fol(cid:173)
`lowing administration of single oral doses of up to
`200 mg to healthy volunteers. The main adverse
`events reported following doses of 90 mg and
`
`6000
`
`5500
`
`5000
`
`4500
`
`4000
`
`:::;-
`E :c
`3500
`ci> .s 3000
`u 2500
`::>
`2000
`<(
`
`1500
`
`1000
`
`500
`
`0
`
`y = 23.4x - 197
`
`r2 = 0.99
`
`!
`
`0
`
`25
`
`50
`
`75
`
`100 125 150 175 200
`Dose (mg)
`
`time
`Figure 3 Mean area under the plas ma concentra tion -
`curve (AUC) values for sildenafi.l following single oral admin(cid:173)
`is tration to h ealthy male volunteers. Each point represents
`mean ± SD from 9 or 10 subj ects
`
`Tab le 2 TC 50 values for inhibition of human POE isozymes by sildenafil. PDEZ , 3 and 5 were isolated from corpu s cavernosum,
`PDEl from cardiac ventricle and PDE4 from s keletal muscle. IC 50 valu es were determined u sin g cGMP (0.5 pM) as substrate for
`PDE1, 2 and 5 and cAMP (0.5 pM) as sub s tra te for PDE3 and 4. Data shown are means ± S.E.M.
`
`Compound
`
`Sildenafil
`
`PDE1
`0.29 ± 0.03
`(n = 7)
`
`PDE2
`
`> 30
`(n = 4)
`
`I C 50 (~1M)
`
`PDE3
`17 ± 3
`(n = 4)
`
`PDE4
`7.3 ± 0.8
`(n = 3)
`
`PDE5
`0.0039 ± 0.009
`(n = 15)
`
`PDE = phosphodies terases
`cGMP = cyclic guano sine monopho s phate
`cAMP =cyclic adenine monopho s pha te
`
`ATI 1023-0004
`
`

`

`above were headache and facial flushing . These
`adverse events were of mild to moderate severity
`and resolved spontaneously after a few hours.
`There were no treatment-related discontinuations.
`
`Clinical efficacy
`For the 12 patients entering the clinical study, the
`mean age was 48 years (range 36 to 63) and the
`mean duration of penile erectile dysfunction 3.4
`years (range 1.5 to 10).
`Two patients were excluded from the ANOV A
`analysis of mean duration of rigidity of greater
`than 60% at the base and tip of the penis. One
`patient had an erection of greater than 60% rigid(cid:173)
`ity which started prior to VSS in the period when
`he received 25 mg of sildenafil. The second patient
`was excluded because penile plethysmography
`was not recorded due to technical difficulties .
`The duration of rigidity of greater than 60'Yo at
`the base and tip of the penis during VSS was sig(cid:173)
`nificantly higher in each treatment group com(cid:173)
`pared with placebo (Figure 4). The geometric mean
`duration (in min) of rigidity of greater than 60% at
`
`**
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`'2
`I
`E
`i=
`
`Q)
`
`Placebo
`
`I 0 mg
`
`25 mg
`
`50 mg
`
`• P=0.00 1 vs placebo
`·· P-:0.003 vs placebo
`
`Dose of sildenafil
`
`(a)
`
`(b)
`
`70
`
`60
`
`50
`
`4o
`
`30
`
`20
`
`10
`
`'2 I
`E
`i=
`
`Q)
`
`Placebo
`
`1 0 mg
`
`25 mg
`
`50 mg
`
`Dose of sildenafil
`
`· P=O.lXH vs placebo
`Figure 4 The mean (95 'Yo CI) duration of rigidity > 60°/t,
`during VSS (a) a t bas e and (b) a t tip of penis in patients who
`received placebo and different do ses of sildenafil (10, 25 and
`50mg)
`
`51
`
`Sildenafil
`M Boolell et a/
`
`the base of the penis was 3.2 (95% confidence
`interval 1.1 to 7.9) on placebo, 25 .9 (11 .7 to 56.8,
`P = 0.001) on 10 mg, 24.1 (10.3 to 55.8, P = 0.003)
`on 25 mg and 31.8 (14.4 to 69.6 , P = 0.001) on
`50 mg of sildenafil. The corresponding values at
`the tip of the penis were 3.0 (95% confidence
`interval 1.3 to 6.4) on placebo, 19.1 (9 .8 to 36.8,
`P = 0.001) on 10mg, 26.3 (13 .0 to 52.7, P = 0.001)
`on 25 mg and 26.5 (13 .7 to 50.8, P = 0.001) on
`50 mg of sildenafil. The onset of penile tumescence
`in all the patients was within the first few
`minutes of commencing VSS or approximately 30
`to 40 min post-dosing with sildenafil, correspond(cid:173)
`ing approximately with the peak plasma concen(cid:173)
`tration of the compound.
`Four patients (two at 25 mg and two at 50 mg)
`reported mild headache. There was no discontin(cid:173)
`uation from the study due to these adverse events,
`which were mild and transient. Importantly, there
`were no significant changes in pulse rate, blood
`pressure or
`laboratory safety data
`in
`these
`patients.
`
`Discussion and conclusion
`
`During recent years it has become increasingly
`clear that the nitric oxide-cGMP system plays a
`key role in the local mechanism of penile erec(cid:173)
`tion. s- s We have demonstrated the presence of
`PDE2, 3 and 5 in human corpora cavernosa, and
`shown that sildenafil is a potent selective inhibi(cid:173)
`tor of human PDE5. Furthermore, the clinical data
`obtained with sildenafil highlight the important
`role of PDE5 in human penile erection.
`Our finding that cGMP-specific PDE5 was the
`major cGMP phosphodiesterase activity isolated
`from human corpus cavernosum and that cGMP(cid:173)
`inhibited PDE3 was also present is in agreement
`with the report of Stieff et al. 18 In general, tissues
`such as corpus cavernosal smooth muscle, in
`which cGMP and cAMP appear to have similar
`physiological effects, express PDE3 in addition to
`the cGMP-specific phosophodiesterase. 19
`20 This
`·
`allows cGMP and cAMP to work synergistically in
`the tissue. 20 We have previously demonstrated
`that
`sildenafil
`potentiates
`relaxation
`of
`phenylephrine-contracted human corpus caver(cid:173)
`nosum elicited by electrical field stimulation, 21
`and endothelium-dependent relaxation of rabbit
`cavernosal
`tissue
`induced by
`the muscarinic
`agonist, methacholine. 22 In both cases the effects
`of sildenafil were dependent on activation of the
`nitric oxide-cGMP mediated relaxation pathway.
`This may explain why Stieff et al. 18
`, who exam(cid:173)
`ined relaxation of cavernosal
`tissue without
`stimulation by nitric oxide, found that the selec(cid:173)
`tive PDE5 inhibitor, zaprinast, was a weaker and
`less effective relaxant of human cavernosal tissue
`than PDE3-selective agents.
`type 5 cGMP(cid:173)
`the
`By selectively
`inhibiting
`improves
`specific phosphodiesterase, sildenafil
`
`ATI 1023-0005
`
`

`

`Sildenafil
`M Boolell et at
`
`penile erectile activity in patients with erectile
`dysfunction during sexual s timulation. Experi(cid:173)
`mental evidence indicates that this
`is a
`local
`action of sildenafil on the corpus cavernosal
`smooth muscle cells where it enhances nitric oxide
`mediated relaxation. 21
`22 We believe that under
`'
`conditions of nitric oxide drive, type 5 PDE is the
`most important regulator of cGMP levels in the
`corpus cavernosal smooth muscle cell. The mecha(cid:173)
`nism of action of sildenafil suggests that it will
`induce or enhance erections during sexual stimu(cid:173)
`lation, but will not provide erections
`in
`the
`absence of any physiological or sexual stimulus.
`Sildenafil demonstrates ideal pharmacokinetics
`for an oral agent to be taken, as required, prior to
`sexual activity for the treatment of penile erectile
`dysfunction. It is rapidly absorbed when adminis(cid:173)
`tered orally, and has an onset of action of less
`t~an one hour. Furthermore, the drug has a rela(cid:173)
`tively. short plasma half life of approximately 4 h
`and IS not expected to accumulate on repeated
`single daily administration. In the dose range
`studied, sildenafil is generally well tolerated and
`has no significant effects on pulse rate and blood
`pressure.
`Sildenafil represents a new class of orally active
`and peripherally acting drug for the treatment of
`penile erectile dysfunction. The RigiScan study
`demonstrates that sildenafil enhances the erectile
`response to erotic stimuli under standardised con(cid:173)
`ditions . The results are reported in terms of mean
`duration of rigidity of greater than 60% . Although
`there is considerable debate as to the threshold
`degree of rigidity which correlates with sufficient
`hardness
`for penetrative
`intercourse, previous
`studies indicate that the vast majority of patients
`who achieve 60% rigidity will have an erection
`24 Its
`that is sufficient for sexual intercourse.23
`'
`efficacy in enhancing erections during stimulation
`in the natural and private setting and its safety on
`long-term adminstration are currently being inves(cid:173)
`tigated in large-scale clinical trials.
`
`Acknowledgements
`We wish to thank Mrs Lucy Short for secretarial assist(cid:173)
`ance and Ms J Pearson for help with the statistical
`analyses.
`
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`
`ATI 1023-0006
`
`