Design, synthesis, and in-silico study of novel triarylethylene analogs with dual anti-estrogenic and serotonergic activity.

Estrogen receptor is an important target in breast cancer. Serotonin receptors (5-HT2A and 5-HT2C , in particular) were investigated for a potential role in development and progression of breast cancer. Ligands that interact with estrogenic receptors influence the emotional state of females. Thus, designing selective estrogen receptor modulator (SERM) analogs with potential serotonergic activity is a plausible approach. The dual ligands can augment cytotoxic effect of SERMs, help in both physical and emotional menopausal symptom relief, enhance cognitive function and support bone health. Herein, we report triarylethylene analogs as potential candidates for treatment of breast cancer. Compound 2e showed (ERα relative ß- galactosidase activity = 0.70), 5-HT2A (Ki = 0.97 µM), and 5-HT2C (Ki = 3.86 µM). It was more potent on both MCF-7 (GI50 = 0.27 µM) and on MDA-MB-231 (GI50 = 1.86 µM) compared to tamoxifen (TAM). Compound 4e showed 40 times higher antiproliferative activity on MCF-7 and 15 times on MDA-MBA compared to TAM. Compound 4e had higher average potency than TAM on all nine tested cell line panels. Our in-silico model revealed the binding interactions of compounds 2 and 2e in the three receptors; further structural modifications are suggested to optimize binding to the ERα, 5-HT2A , and 5-HT2C .

Obese Male Mice Exposed to Early Life Stress Display Sympathetic Activation and Hypertension Independent of Circulating Angiotensin II.

BACKGROUND: We have previously reported that male mice exposed to maternal separation and early weaning (MSEW), a model of early life stress, show sympathetic activation and increased blood pressure in response to a chronic high-fat diet. The goal of this study was to investigate the contribution of the renin-angiotensin-aldosterone system to the mechanism by which MSEW increases blood pressure and vasomotor sympathetic tone in obese male mice. METHODS AND RESULTS: Mice were exposed to MSEW during postnatal life. Undisturbed litters served as controls. At weaning, both control and MSEW offspring were placed on a low-fat diet or a high-fat diet for 20 weeks. Angiotensin peptides in serum were similar in control and MSEW mice regardless of the diet. However, a high-fat diet induced a similar increase in angiotensinogen levels in serum, renal cortex, liver, and fat in both control and MSEW mice. No evidence of renin-angiotensin system activation was found in adipose tissue and renal cortex. After chronic treatment with enalapril (2.5 mg/kg per day, drinking water, 7 days), an angiotensin-converting enzyme inhibitor that does not cross the blood-brain barrier, induced a similar reduction in blood pressure in both groups, while the vasomotor sympathetic tone remained increased in obese MSEW mice. In addition, acute boluses of angiotensin II (1, 10, 50 µg/kg s.c.) exerted a similar pressor response in MSEW and control mice before and after enalapril treatment. CONCLUSIONS: Overall, elevated blood pressure and vasomotor sympathetic tone remained exacerbated in MSEW mice compared with controls after the peripheral inhibition of angiotensin-converting enzyme, suggesting a mechanism independent of angiotensin II.

Development of N-Terminally Modified Variants of the CXCR4-Antagonistic Peptide EPI-X4 for Enhanced Plasma Stability.

EPI-X4, a natural peptide CXCR4 antagonist, shows potential for treating inflammation and cancer, but its short plasma stability limits its clinical application. We aimed to improve the plasma stability of EPI-X4 analogues without compromising CXCR4 antagonism. Our findings revealed that only the peptide N-terminus is prone to degradation. Consequently, incorporating d-amino acids or acetyl groups in this region enhanced peptide stability in plasma. Notably, EPI-X4 leads 5, 27, and 28 not only retained their CXCR4 binding and antagonism but also remained stable in plasma for over 8 h. Molecular dynamic simulations showed that these modified analogues bind similarly to CXCR4 as the original peptide. To further increase their systemic half-lives, we conjugated these stabilized analogues with large polymers and albumin binders. These advances highlight the potential of the optimized EPI-X4 analogues as promising CXCR4-targeted therapeutics and set the stage for more detailed preclinical assessments.

Structure-Activity Relationships of Triphenylethylene Derivatives and Their Evaluation as Anticancer and Antiviral Agents.

Tamoxifen (TAM) is a selective estrogen receptor modulator (SERM) that is used in the treatment of breast cancer, yet with the risk of developing uterine cancer. A perfect SERM would act as an estrogen activator on bones, the cardiovascular system, and the central nervous system while providing neutral or estrogen blocking effects on the breast and the uterus. Herein, we report on the design, synthesis, and evaluation of new rigid and flexible TAM analogues. Mainly, a chloro substituent is introduced at the para position of the TAM ring C blocking the CYP2D6 hydroxylation site. Most compounds showed estrogenic activity higher than TAM using the yeast estrogen screen assays, indicating the determinant role of the chloro substituent upon functional activity. Despite being estrogenic, compound 2B showed potent antiproliferative activity in the NCI 60 cell lines with mean GI50 = 3.67 µM, GI50 = 1.05 µM on MCF-7 cell lines, and GI50 = 1.30 µM on MDA-MB-231. The estrogenic activity of compound 2B was further confirmed by stimulating alkaline phosphatase in Ishikawa cells, and it showed no increase in relative uterine wet weight in ovariectomized rats. Compound 2F showed EC90 = 0.31 µg/mL and SI90 = 60 against Ebola virus; this is 200-fold more potent than the positive control favipiravir. This is the first time to report estrogenic triphenylethylenes as anti-EBOV agents. The anti-EBOV activity reported is a function of the substitution pattern of the scaffold rather than the functional activity. Moreover, compound 3D showed excellent PO pharmacokinetic properties in mice. In conclusion, for this class of TAM-like compounds, the blockage of the p-position of ring C is decisive for the functional activity; meanwhile, the triarylethylene substitution pattern is detrimental for the antiviral activity.

Design, synthesis, and metabolite identification of Tamoxifen esterase-activatable prodrugs.

Tamoxifen (TAM) is used in treatment of hormonal dependent breast cancer, both in premenopausal and postmenopausal women. TAM is intrinsically metabolized by CYP450 enzymes to more active metabolites. Recent reports identified CYP2D6, an enzyme involved in the conversion of TAM to the more potent 4-OH-TAM, is encoded by theCYP2D6gene, which is highly polymorphic. Women with inactive alleles are poor metabolizers; in many cases they suffer acquired TAM resistance. Herein we report synthesis and biological evaluation of novel TAM analogues. The novel analogues are designed to elude CYP2D6 metabolism. Hydrolysis of the carbamate moiety on ring C is mediated via carboxylesterases. Compound 3d [E/Z Benzyl-carbamic acid4-{2-benzyl-1-[4-(2-pyrrolidin-1-yl-ethoxy)-phenyl]-but-1-enyl}-phenyl ester] showed GI50 = 0.09 µM on MCF-7 and GI50 = 1.84 µM on MDA-MB231 cell lines. To further validate our hypothesis, metabolites of selected novel analogues were determined in vitro under different incubation conditions. The hydroxylated analogues were obtained under non CYP2D6 dependent conditions. Compound 8d, a benzyl carbamate derivative, was the least-stable analog and showed the highest rate of metabolism among all tested analogues. Our in silico model showed the novel flexible analogues can still adopt an antiestrogenic binding profile occupying the same pocket as 4-OH-TAM.

Flexible Etherified and Esterified Triphenylethylene Derivatives and Their Evaluation on ER-positive and Triple-Negative Breast Cancer Cell Lines.

Tamoxifen (TAM) is a selective estrogen receptor modulator (SERM) with potential clinical benefits for all stages of breast cancer. TAM is primarily metabolized to more potent metabolites via polymorphic CYP2D6. This affects the clinical outcome of TAM treatment. Herein we report novel TAM analogues that can avoid metabolism via CYP2D6. The novel analogues bear a flexible skeleton. Compounds have either an ester group on ring C or homodiaminoalkoxy groups on rings B and C. Compound 6 (E/Z-4-[1-[4-(2-diethylaminoethoxy)phenyl]-3-(4-methoxyphenyl)-2-methyl[propenyl]phenol) was found to be ten-fold more potent than TAM on MCF-7 cells (GI50 =0.15 µM). It showed fivefold greater inhibitory activity on MDA-MB-231 cells than TAM (GI50 =1.71 µM). Compound 13 (4-{3,3-bis-[4-(3-dimethylaminopropoxy)phenyl]-2-methylallyl}methoxybenzene) was the most potent among the homodiaminoalkoxy derivatives (GI50 =0.44) on both MCF-7 and MDA-MB-231 cell lines, respectively. Furthermore, the COMPARE algorithm suggested that it has different molecular targets from those of some other reported anticancer drugs.

Manipulating Estrogenic/Anti-Estrogenic Activity of Triphenylethylenes towards Development of Novel Anti-Neoplastic SERMs.

Selective estrogen receptor modulators (SERMs) act as estrogen receptor (ERα) agonists or antagonists depending on the target issue. Tamoxifen (TAM) (a non-steroidal triphenylethylene derivative) was the first SERM approved as anti-estrogen for the treatment of metastatic breast cancer. On the hunt for novel SERMs with potential growth inhibitory activity on breast cancer cell lines yet no potential to induce endometrial carcinoma, we designed and synthesized 28 novel TAM analogs. The novel analogs bear a triphenylethylene scaffold. Modifications on rings A, B, and C aim to attenuate estrogenic/anti-estrogenic activities of the novel compounds so they can potentially inhibit breast cancer and provide positive, beneficial estrogenic effects on other tissues with no risk of developing endometrial hyperplasia. Compound 12 (E/Z-1-(2-{4-[1-(4-Chloro-phenyl)-2-(4-methoxy-phenyl)-propenyl]-phenoxy}-ethyl)-piperidine) showed an appreciable relative ERα agonistic activity in a yeast estrogen screen (YES) assay. It successfully inhibited the growth of the MCF-7 cell line with GI50 = 0.6 µM, and it was approximately three times more potent than TAM. It showed no potential estrogenicity on Ishikawa endometrial adenocarcinoma cell line via assaying alkaline phosphatase (AlkP) activity. Compound 12 was tested in vivo to assess its estrogenic properties in an uterotrophic assay in an ovariectomized rat model. Compared to TAM, it induced less increase in wet uterine wet weight and showed no uterotrophic effect. Compound 12 is a promising candidate for further development due to its inhibition activity on MCF-7 proliferation with moderate AlkP activity and no potential uterotrophic effects. The in vitro estrogenic activity encourages further investigations toward potential beneficial properties in cardiovascular, bone, and brain tissues.

Stability-indicating UPLC, TLC-densitometric and UV-spectrophotometric methods for alcaftadine determination.

Three sensitive and precise stability-indicating methods were developed for the determination of alcaftadine in the presence of its degradation products. Efficient separation was achieved using UPLC-UV-MS method by gradient elution with a mobile phase of 0.1% aqueous formic acid (A) and 0.1% formic acid in acetonitrile (B) over concentration range of 0.10-1.00 µg mL-1. The accuracy was 100.89% ± 0.74 and 99.73% ± 0.78 for UV and MS detection, respectively. A TLC-densitometric method was adopted to separate of the intact drug from its degradation products. Methanol: chloroform: glacial acetic acid (5:4:0.1, v/v/v) was the developing system, detection wavelength was set to 282 nm. Rf values were 0.35, 0.65 and 0.88 for alcaftadine, its acidic and oxidative degradants, respectively. The linearity range was 2.00-27.00 µg/band with mean accuracy of 100.58% ± 0.86. The proposed TLC-densitometric method was utilized for the study of degradation rates of alcaftadine. Finally, a simple UV-spectrophotometric method where an induced dual wavelength was implemented, the method showed a linearity range of 2.00-27.00 µg mL-1 with mean recovery of 100.15% ± 0.70. The proposed methods were successful for quantitation of alcaftadine in ophthalmic solution and in plasma samples. The obtained results were in accordance with those obtained by previously reported methods.

Design and Synthesis of Novel Symmetric Fluorene-2,7-Diamine Derivatives as Potent Hepatitis C Virus Inhibitors.

Hepatitis C virus (HCV) is an international challenge. Since the discovery of NS5A direct-acting antivirals, researchers turned their attention to pursue novel NS5A inhibitors with optimized design and structure. Herein we explore highly potent hepatitis C virus (HCV) NS5A inhibitors; the novel analogs share a common symmetrical prolinamide 2,7-diaminofluorene scaffold. Modification of the 2,7-diaminofluorene backbone included the use of (S)-prolinamide or its isostere (S,R)-piperidine-3-caboxamide, both bearing different amino acid residues with terminal carbamate groups. Compound 26 exhibited potent inhibitory activity against HCV genotype (GT) 1b (effective concentration (EC50) = 36 pM and a selectivity index of >2.78 × 106). Compound 26 showed high selectivity on GT 1b versus GT 4a. Interestingly, it showed a significant antiviral effect against GT 3a (EC50 = 1.2 nM). The structure-activity relationship (SAR) analysis revealed that picomolar inhibitory activity was attained with the use of S-prolinamide capped with R- isoleucine or R-phenylglycine residues bearing a terminal alkyl carbamate group.

Tamoxifen and oxidative stress: an overlooked connection.

Tamoxifen is the gold standard drug for the treatment of breast cancer in pre and post-menopausal women. Its journey from a failing contraceptive to a blockbuster is an example of pharmaceutical innovation challenges. Tamoxifen has a wide range of pharmacological activities; a drug that was initially thought to work via a simple Estrogen receptor (ER) mechanism was proven to mediate its activity through several non-ER mechanisms. Here in we review the previous literature describing ER and non-ER targets of tamoxifen, we highlighted the overlooked connection between tamoxifen, tamoxifen apoptotic effects and oxidative stress.

Symmetric benzidine derivatives as anti-HCV agents: Insight into the nature, stereochemistry of the capping amino acid and the size of the terminal capping carbamates.

Novel symmetric molecules, bearing a benzidine prolinamide core, two terminal carbamate caps of variable sizes and nature, including natural and unnatural amino acids were developed. Several terminal N-carbamate substituents of the core structure, ranging from linear methyl, ethyl and butyl groups to branching isobutyl group; and an aromatic substituent were also synthesized. Series 1 has hydrophobic AA residues, namely S and R phenylglycine and a terminal carbamate capping group, whereas Series 2 bears sulphur containing amino acids, specifically S and R methionine and the natural R methylcysteine. The novel compounds were tested for their inhibitory activity (EC50) and their cytotoxicity (CC50), using an HCV 1b (Con1) reporter replicon cell line. Compound 4 with the unnatural capping residue, bearing d-Phenylglycine amino acid residue and N-isobutyloxycarbonyl capping group, was the most active within the two series, with EC50 = 0.0067 nM. Moreover, it showed high SI50 > 14788524 and was not cytotoxic at the highest tested concentration (100 µΜ), indicating its safety profile. Compound 4 also inhibited HCV genotypes 2a, 3a and 4a. Compared to the clinically approved NS5A inhibitor Daclatasvir, compound 4 shows higher activity against genotypes 1b and 3a, as well as improved safety profile.

Extending the use of tadalafil scaffold: Development of novel selective phosphodiesterase 5 inhibitors and histone deacetylase inhibitors.

Herein we present the synthesis and characterization of a novel chemical series of tadalafil analogues that display different pharmacological profiles. Compounds that have the 6R, 12aR configuration and terminal carboxylic acid group at the side chain arising from the piperazinedione nitrogen were potent PDE5 inhibitors, with compound 11 having almost equal potency to tadalafil and superior selectivity over PDE11, the most common off-target for tadalafil. Modifying the stereochemistry into 6S, 12aS configuration and adopting the hydroxamic acid moiety as a terminal group gave rise to compounds that only inhibited HDAC. Dual PDE5/HDAC inhibition could be achieved with compounds having 6R, 12aR configuration and hydroxamic acid moiety as a terminal group. The anticancer activity of the synthesized compounds was evaluated against a diverse number of cell lines of different origin. The compounds elicited anticancer activity against cell lines belonging to lymphoproliferative cancer as well as solid tumors. Despite the previous reports suggesting anticancer activity of PDE5 inhibitors, the growth inhibitory activity of the compounds seemed to be solely dependent on HDAC inhibition. Compound 26 (pan HDAC IC50 = 14 nM, PDE5 IC50 = 46 nM) displayed the most potent anticancer activity in the present series and was shown to induce apoptosis in Molt-4 cells. HDAC isoform selectivity testing for compound 26 showed that it is more selective for HDAC6 and 8 over HDAC1 by more than 20-fold.

Synthesis of novel flexible tamoxifen analogues to overcome CYP2D6 polymorphism and their biological evaluation on MCF-7 cell line.

Tamoxifen (TAM) is currently the endocrine treatment of choice for all stages of breast cancer; it has proven success in ER positive and ER negative patients. TAM is activated by endogenous CYP450 enzymes to the more biologically active metabolites 4-hydroxytamoxifen and endoxifen mainly via CYP2D6 and CYP3A4/5. CYP2D6 has been investigated for polymorphism; there is a large interindividual variation in the enzyme activity, this drastically effects clinical outcomes of tamoxifen treatment. Here in we report the design and synthesis of 10 novel compounds bearing a modified tamoxifen skeleton, ring C is substituted with different ester groups to bypass the CYP2D6 enzyme metabolism and employ esterase enzymes for activation. All compounds endorse flexibility on ring A. Compounds (II-X) showed MCF-7% growth inhibition >50% at a screening dose of 10 µM. These results were validated by yeast estrogen screen (YES) and E-Screen assay combined with XTT assay. Compound II (E/Z 4-[1-4-(3-Dimethylamino-propoxy)-phenyl)-3-(4-methoxy-phenyl)-2-methyl-propenyl]-phenol) showed nanomolar antiestrogenic activity (IC50 = 510 nM in YES assay) and was five times more potent in inhibiting the growth of MCF-7 BUS (IC50 = 96 nM) compared to TAM (IC50 = 503 nM). Esterified analogues VI, VII were three times more active than TAM on MCF-7 BUS (IC50 = 167 nM). Novel analogues are prodrugs that can ensure equal clinical outcomes to all breast cancer patients.

Tadalafil: 15 years' journey in male erectile dysfunction and beyond.

Hit, Lead & Candidate Discovery Tadalafil, Cialis, Eli Lilly & Co./ICOS, (6R,12aR)-6-(1,3-benzodioxol-5-yl)-2-methyl-2,3,6,7,12,12a-hexahydropyrazino[1',2':1,6] pyrido[3,4-b]indole-1,4-dione, was first discovered in 2003. It was reported to have high diastereospecificity for phosphodiesterase 5 (PDE5) inhibitions. The cis-(6R, 12aR) enantiomer is the most active enantiomer. Tadalafil showed PDE5 inhibition with IC50 = 5 nM. It possesses high selectivity for PDE5 versus PDE1-4 and PDE6. Tadalafil is more selective to PDE5 against PDE6 whereas sildenafil, another commercially available PDE5 inhibitor shows similar potencies to inhibit PDE5 and PDE6. Tadalafil is used for the treatment of male erectile dysfunction (MED), prostatic benign hyperplasia (PBH) signs and symptoms, and pulmonary arterial hypertension (PAH). Adcirca, another name for tadalafil, is used to treat PAH and improve exercise capacity. Recent clinical studies suggest the use of tadalafil for nonurological applications, including circulatory disorders (ischemia injury, myocardial infarction, cardiac hypertrophy, cardiomyopathy, heart failure, and stroke), neurodegenerative disorders, and cognitive impairment conditions. This review discusses tadalafil and its analogues reported in the past 15 years. It discusses synthetic pathways, structural activity relationships, existing and future pharmacological indications of tadalafil and its analogues. This work can help medicinal chemists developing novel PDE5 inhibitors with wider therapeutic indications.

Expanding the chemical space of anti-HCV NS5A inhibitors by stereochemical exchange and peptidomimetic approaches.

Here we report a series of potent anti-HCV agents bearing a symmetrical benzidine l-prolinamide backbone with different capping groups including alkyl/aryl carbamates of natural and unnatural valine and leucine amino acids. All compounds were investigated for their inhibitory activity in an HCV replicon assay on genotype 1b. The novel compounds share some chemical and clinical attributes of commercially available NS5A inhibitors. Compounds 5 and 6 with unnatural capping residue and ethyl and isobutyl carbamates showed EC50 values in the picomolar range with a low toxicity profile and selectivity indices of several orders of magnitude. These findings enlarge the chemical space from which NS5A inhibitors may be discovered by adopting unnatural amino acids, amino acids other than valine and carbamates other than methyl as the capping groups.

Design and synthesis of novel flexible ester-containing analogs of tamoxifen and their evaluation as anticancer agents.

BACKGROUND: Tamoxifen (TAM) is metabolized to the more active 4-hydroxytamoxifen by CYP2D6 enzyme. Due to the genetic polymorphisms in CYP2D6, clinical outcomes of TAM treatment vary. Novel flexible TAM analogs with altered activation pathway were synthesized and were tested for their antiproliferative action on MCF-7 cell lines and their binding affinity for ERα and ERß. RESULTS: All compounds showed better antiproliferative activity than TAM. Compound 3 showed 80-times more ERα binding than TAM, 900-times more selectivity toward ERα. Compound 3 was tested on the entire National Cancer Institute cancerous cell lines; results indicated a broad spectrum anticancer activity. CONCLUSION: The novel analogs were more potent than TAM with higher selectivity toward ERα and with potential metabolic stability toward CYP2D6.

Design and synthesis of novel tamoxifen analogues that avoid CYP2D6 metabolism.

Tamoxifen (TAM) is a widely used drug in the prophylaxis and treatment of breast cancer. TAM is metabolized to the more active 4-hydroxytamoxifen (4-OH-TAM) and endoxifen by cytochrome P450 (CYP) mainly CYP2D6 and CYP3A4 enzymes. Due to the genetic polymorphisms in CYP2D6 genes, high variation in the clinical outcomes of TAM treatment is observed among women of different populations. To address this issue, novel TAM analogues with possible altered activation pathways were synthesized. These analogues were tested for their antiproliferative action on MCF-7 breast cancer cell lines as well as their binding affinity for estrogen receptor (ER) ER-α and ER-ß receptors. These entire novel compounds showed better antiproliferative activity than did TAM on the MCF-7 cells. Moreover, compound 10 exhibited a half maximal growth inhibition (GI50) that was 1000 times more potent than that of TAM (GI50 < 0.005 µM vs 1.58 µM, respectively). Along with a broad spectrum activity on various cancer cell lines, all the TAM analogues showed considerable activity on the ER-negative breast cancer cell line. For further study, compound 10 was incubated in human liver microsomes (HLM), human hepatocytes (hHEP) and CYP2D6 supersomes. The active hydroxyl metabolite was detected after incubation in HLM and hHEP, implicating the involvement of other enzymes in its metabolism. These results prove that this novel series of TAM analogues might provide improved clinical outcomes for poor 2D6 metabolizers.

Structure-Based Design of Novel Tetrahydro-Beta-Carboline Derivatives with a Hydrophilic Side Chain as Potential Phosphodiesterase Inhibitors.

Tadalafil is a clinically approved phosphodiesterase-5 inhibitor for the treatment of erectile dysfunction and pulmonary arterial hypertension. It contains two chiral carbons, and the marketed isomer is the 6R, 12aR isomer with a methyl substituent on the terminal nitrogen of the piperazinedione ring. In this report, tadalafil analogues with an extended hydrophilic side chain on the piperazine nitrogen were designed to interact with particular hydrophilic residues in the binding pocket. This leads to analogues with moderate inhibitory activity on phosphodiesterase-5, even for isomers in which chiral carbons are of the S configuration.

Discovery of a novel series of tetrahydro-ß-carbolines inducing autophagic cell death in human metastatic melanoma.

Herein, we report the synthesis of novel tetrahydro-ß-carbolines that induce cell death via the autophagic pathway. Five of the new compounds induced cell death in a panel of patient-derived human metastatic melanoma cells. The autophagic pathway was confirmed using LC3 autophagosome markers; the involvement of ATG7 and Beclin 1 autophagy regulating genes was confirmed using infection with short hairpin RNA (shRNA) to silence Beclin 1 and ATG7. Compound VIII (IC50 = 2.34-5.15 µM) displayed activities greater than cisplatin against a panel of patient-derived human metastatic melanoma cell lines. The structure-activity relationship (SAR) of this class and the role of the absolute stereochemistry and geometrical isomerism are evaluated.

Design of novel ß-carboline derivatives with pendant 5-bromothienyl and their evaluation as phosphodiesterase-5 inhibitors.

New derivatives with the tetrahydro-ß-carboline-imidazolidinedione and tetrahydro-ß-carboline-piperazinedione scaffolds and a pendant bromothienyl moiety at C-5/C-6 were synthesized and tested for their ability to inhibit PDE5 in vitro. The following SAR can be concluded: The tetracyclic scaffold is essential for PDE5 inhibition; the ethyl group is the most suitable among the adopted N-substituents on the terminal ring (hydantoin/piperazinedione); the appropriate stereochemistry of C-5/C-6 derived from the aldehyde rather than C-11a/C-12a derived from tryptophan appears crucial for inhibition of PDE5; surprisingly, derivatives with the hydantoin terminal ring are more active than their analogs with the piperazinedione ring; the selectivity versus PDE5 relative to PDE11 with cGMP as a substrate is mainly a function of the substitution and stereochemistry pattern of the external ring, in other words of the interaction with the H-loop residues of the isozymes. Thirteen derivatives showed PDE5 inhibitory activity with IC(50) values in the range of 0.16-5.4 µm. Compound 8 was the most potent PDE5 inhibitor and showed selectivity towards PDE5 versus other PDEs, with a selectivity index of 49 towards PDE5 rather than PDE11 with cGMP as the substrate.