Pyrimidine and triazine 3-oxide sulfates: a new family of vasodilators

Human Vascular Wall Microfluidic Model for Preclinical Evaluation of Drug-Induced Vascular Injury

Drug-induced vascular injury (DIVI) in preclinical animal models often leads to candidate compound termination during drug development. DIVI has not been documented in human clinical trials with drugs that cause DIVI in preclinical animals. A robust human preclinical assay for DIVI is needed as an early vascular injury screen. A human vascular wall microfluidic tissue chip was developed with a human umbilical vein endothelial cell (HUVEC)-umbilical artery smooth muscle cell (vascular smooth muscle cell, VSMC) bilayer matured under physiological shear stress. Optimized temporal flow profiles produced HUVEC-VSMC bilayers with quiescent endothelial cell (EC) monolayers, EC tight junctions, and contractile VSMC morphology. Dose-response testing (3-30 μM concentration) was conducted with minoxidil and tadalafil vasodilators. Both drugs have demonstrated preclinical DIVI but lack clinical evidence. The permeability of severely damaged engineered bilayers (30 μM tadalafil) was 4.1 times that of the untreated controls. Immunohistochemical protein assays revealed contrasting perspectives on tadalafil and minoxidil-induced damage. Tadalafil impacted the endothelial monolayer with minor injury to the contractile VSMCs, whereas minoxidil demonstrated minor EC barrier injury but damaged VSMCs and activated ECs in a dose-response manner. This proof-of-concept human vascular wall bilayer model of DIVI is a critical step toward developing a preclinical human screening assay for drug development. Impact statement More than 90% of drug candidates fail during clinical trials due to human efficacy and toxicity concerns. Preclinical studies rely heavily on animal models, although animal toxicity and drug metabolism responses often differ from humans. During the drug development process, perfused in vitro human tissue chips could model the clinical drug response and potential toxicity of candidate compounds. Our long-term objective is to develop a human vascular wall tissue chip to screen for drug-induced vascular injury. Its application could ultimately reduce drug development delays and costs, and improve patient safety.

Prolonged hypotension induced by ingesting a topical minoxidil solution: analysis of minoxidil and its metabolites

Case

A 47-year-old man ingested 60 mL of a topical solution (3,000 mg minoxidil) and presented with prolonged hypotension. Treatment with dopamine hydrochloride and noradrenaline provided blood pressure control. Serum unchanged minoxidil concentrations at 4 and 16 h after ingestion were 4,994 and 33.9 ng/mL, respectively. Urine concentrations of unchanged minoxidil, minoxidil-O-glucuronide, and minoxidil-N-O-sulfate at 16 h after ingestion were 360.4, 1,953, and 104.5 ng/mL, respectively.

Outcome

The serum unchanged minoxidil concentration rapidly decreased over a short interval. However, the patient needed to receive vasopressor support for the first 4 days after being admitted to the hospital. The urine minoxidil-O-glucuronide concentration was higher than the concentrations of unchanged minoxidil and minoxidil-N-O-sulfate.

Conclusion

Although the serum concentration of unchanged minoxidil rapidly decreased, ingesting large amounts of a topical minoxidil solution can have serious and prolonged cardiovascular effects. Analyzing the minoxidil-O-glucuronide concentration in urine is useful for diagnosing minoxidil poisoning.

Possible Bioactivation Pathways of Lamotrigine

The anticonvulsant lamotrigine is associated with idiosyncratic drug reactions, especially skin rashes. Most idiosyncratic reactions are believed to be caused by reactive metabolites. Previous studies have found evidence that an arene oxide is formed in rats; however, when we incubated radiolabeled lamotrigine with rat liver microsomes virtually no covalent binding was detected, and the expected downstream phenolic metabolites are not observed in humans. Rare cases of agranulocytosis have been associated with lamotrigine therapy, and we found that lamotrigine is oxidized to two different N-chloro products by HOCl. The more reactive N-chloro metabolite forms an adduct with N-acetylhistidine, and covalent binding was observed when radiolabeled lamotrigine was incubated with myeloperoxidase/H(2)O(2)/Cl(-). Another lamotrigine metabolite is an N-oxide. If this N-oxide were sulfated, it might be sufficiently reactive to bind to protein. The synthetic N-sulfate reacted with N-acetylserine; however, no covalent binding was detected when the radiolabeled N-oxide was incubated with sulfotransferase. We also investigated the possibility that lamotrigine might be oxidized to a free radical by other peroxidases or oxidized by other enzymes such as prostaglandin H synthase or tyrosinase, but no evidence of oxidation was found, and lamotrigine did not cause any detectable increase in lipid peroxidation in vivo. In view of the virtual lack of covalent binding to hepatic microsomes and the lack of any other likely pathway leading to metabolic activation in the skin, it is possible that the parent drug rather than a reactive metabolite causes lamotrigine-induced skin rashes.

Pharmacotherapy for Erectile Dysfunction

INTRODUCTION

Advances in understanding of the biochemistry and physiology of penile erection have led to breakthroughs in pharmacotherapy of erectile dysfunction.

AIM

To provide recommendations/guidelines concerning state-of-the-art knowledge for the putative molecular and cellular mechanisms of action of centrally and peripherally acting drugs currently utilized in pharmacotherapy of erectile dysfunction.

METHODS

An international consultation in collaboration with the major urology and sexual medicine associations assembled over 200 multidisciplinary experts from 60 countries into 17 committees. Committee members established specific objectives and scopes for various male and female sexual medicine topics. The recommendations concerning state-of-the-art knowledge in the respective sexual medicine topic represent the opinion of experts from five continents developed in a process over a two-year period. Concerning the Pharmacotherapy for Erectile Dysfunction Committee there were 25 experts from 10 countries.

MAIN OUTCOME MEASURE

Expert opinion was based on grading of evidence-based medical literature, widespread internal committee discussion, public presentation and debate.

RESULTS

Selective and potent oral PDE5 inhibitors have significantly more affinity than cGMP and form broader molecular interactions with multiple amino acids, thereby blocking access to cGMP in the catalytic sites of the PDE5 enzyme. PDE5 inhibitors, which vary as to biochemical potency, selectivity and pharmacokinetics, lead to cGMP elevation and relaxation facilitation of penile corpus cavernosum smooth muscle cells following sexual stimulation. Various centrally acting drugs influence sexual behaviour. In particular, the dopaminergic substance apomorphine is a central enhancer that acts in the paraventricular nucleus of the hypothalamus as a dopamine (D2) receptor agonist, induces and increases penile erection responses via disinhibition, following sexual stimulation.

CONCLUSIONS

There is a need for more research in the pharmacotherapeutic development of central and peripheral agents for safe and effective erectile dysfunction treatment.

TRANSACTIVATION OF GLUCOCORTICOID-INDUCIBLE RAT ARYL SULFOTRANSFERASE (SULT1A1) GENE TRANSCRIPTION

The purpose of the current study was to establish the role of the glucocorticoid receptor (GR) and androgen receptor (AR) transcription factors in the transactivation of rat aryl sulfotransferase (SULT1A1) gene transcription and to identify the functional hormone-responsive element(s) in the SULT1A1 gene. A cis-acting inverted repeat with three intervening bases (IR3) was identified in the 5'-flanking of the SULT1A1 gene that mediates the transactivation of SULT1A1 gene transcription by both the GR and AR. CV-1 cells were cotransfected with SULT1A1-luciferase reporter plasmids and either wild-type or mutant GR or AR expression constructs. In cotransfectants expressing the wild-type GR, treatment with triamcinolone acetonide produced an approximately 4- to 6-fold induction of luciferase activity in IR3-containing SULT1A1 reporter plasmids. IR3-containing SULT1A1 reporter constructs were also activated by treatment with the synthetic androgen R1881 in cells cotransfected with wild-type but not mutant AR. In primary cultured rat hepatocytes, androgen-inducible expression of IR3-containing SULT1A1 reporter plasmids required cotransfection with AR expression plasmid. Targeted disruption of the SULT1A1 IR3 by mutation of a conserved GT sequence in the 3' half-site of the element ablated GR and AR responsiveness. These results indicate that a proximal IR3 element in the 5'-flanking region of the rat SULT1A1 gene is sufficient for the transactivation of SULT1A1 gene transcription by the GR and AR, and that relative to the GR, functional AR activity is reduced in primary cultured rat hepatocytes.

Topical agents and erectile dysfunction: is there a place?

Despite the proven efficacy of oral therapy for erectile dysfunction (ED), some patients are unable to take these medications because of drug interactions (ie, sildenafil and nitroglycerin) or a lack of response. Topical agents represent another minimally invasive option for the treatment of ED. This review discusses the impediments to effective topical therapy and examines the developmental status of several candidate drugs. Although still in the investigative stage, topical medications can be another tool in the urologist's armamentarium against ED.

Human adult and foetal liver sulphotransferases: inhibition by mefenamic acid and salicylic acid

1. The aim was to see whether mefenamic acid and salicylic acid had different inhibition profiles for SULT1A1 (substrate: 4-nitrophenol) and SULT1A3 (dopamine) activities and on (-)-salbutamol and minoxidil sulphation rates in the human adult and mid-gestational foetal livers. 2. The activity (pmolmin(-1) mg(-1) of SULT1A1 was 662 +/- 78 (adult) and 246 +/- 159 (foetus; p = 0.003) and that of SULT1A3 was 24 +/- 4 (adult) and 121 +/- 90 (foetus; p = 0.030). The rate (pmol min(-1) mg(-1)) of (-)-salbutamol sulphation was 109 +/- 27 (adult) and 117 +/- 34 (foetus; p = (0.144) and that of minoxidil sulphation was 202 +/- 38 (adult) and 108 +/- 44 (foetus; p = 0.001). 3. With mefenamic acid as an inhibitor, the IC50 (microM) for SULT1A1 was 0.2 +/- 0.004 (adult) and 0.01 +/- 0.002 (foetus; p = 0.001); for SULT1A3 it was 76 +/- 6 (adult) and 77 +/- 13 (foetus; p = 0.889); for the rate of ( )-salbutamol sulphation it was 0.07 +/- 0.005 (adult) and not determinable (foetus) and for minoxidil sulphation it was 1.6 +/- 0.7 (adult) and 0.15 +/- 0.04 (foetus; p = 0.076). 4. With salicylic acid as an inhibitor, the IC50 (microM) for SULT1A1 was 30 +/- 2 (adult) and 25 +/- 1 (foetus; p = 0.011); for SULT1A3 it was 690 +/- 36 (adult) and 570 +/- 16 (foetus; p = 0.229); for the rate of ( )-salbutamol sulphation it was 93 +/- 11 (adult) and 344 +/- 42 (foetus; p = 0.010); with minoxidil as substrate, the IC50 was not determinable. 5. In summary, SULT1A1, SULT1A3 and the sulphotransferases towards (-)-salbutamol and minoxidil had measurable activities in the mid-gestational human foetal liver. Mefenamic acid was a more potent inhibitor than salicylic acid of both human adult and foetal liver SULT1A1 and SULT1A3 activities. Foetal liver SULT1A1 was more susceptible than adult liver SULT1A1 to inhibition by mefenamic acid and salicylic acid. These results are consistent with the view that sulphotransferases develop early in the human foetal liver and drugs may inhibit their activities.

Induction of rat hepatic aryl sulfotransferase (SULT1A1) gene expression by triamcinolone acetonide: impact on minoxidil-mediated hypotension

The hypotensive agent minoxidil (6-imino-1, 2-dihydro-1-hydroxy-2-imino-4-piperidinopyrimidine) depends upon aryl sulfotransferase (SULT1)-catalyzed sulfation for its bioactivation. Previous reports suggest that glucocorticoids induce class-specific SULT1 and isoform-specific SULT1A1 gene expression in rat liver. In the present study, rats were treated with the glucocorticoid triamcinolone acetonide (TA, 5 mg/kg/day i.p. x 3 days) or its vehicle, 2% Tween-20, prior to minoxidil, and subsequent effects on mean arterial pressure (MAP), heart rate (HR), and hepatic SULT1 gene expression were characterized. Minoxidil treatment (1.5 mg/kg) resulted in a steady decline in MAP values of 16.3 to 18.6% relative to basal control levels at 35 to 60 min following minoxidil injection. Pentachlorophenol (PCP, 40 micromol/kg i.p.), an inhibitor of SULT1 enzyme activity, effectively ablated the hypotensive effects of minoxidil. By contrast, pretreatment with TA significantly enhanced minoxidil-induced hypotension. Relative to vehicle-treated controls, TA-treated rats displayed a steeper rate of decline in MAP and more profound levels of hypotension with decreases in MAP following minoxidil administration of 27.8%. TA also produced significant increases in hepatic SULT1 mRNA expression (of 271%) and SULT1A1 immunoreactive protein levels (of 273%), relative to vehicle-treated controls. These results provide physiological evidence to support the biological relevance of SULT1A1 induction by glucocorticoids. The data indicate that steroid treatment induces SULT1A1 gene expression and, as a consequence, accentuates the hypotensive effects of minoxidil.

Topical prostaglandin E1 SEPA gel for the treatment of erectile dysfunction

PURPOSE

We compare the systemic effects, local tolerance and effectiveness of topical gel formulations on the penis containing alprostadil (prostaglandin E1) plus 5% SEPA versus SEPA alone (placebo) in men with erectile dysfunction.

MATERIALS AND METHODS

Erectile response, skin discomfort and erythema were measured in 48 men with erectile dysfunction secondary to vascular, neurogenic, psychogenic or mixed etiologies in this single-blind, placebo controlled trial.

RESULTS

Application of prostaglandin E1 gel correlated positively with erectile response as 67 to 75% of patients had an erection compared to 17% of controls (p<0.001). Blood pressure and heart rate varied minimally. No serious adverse effects were observed in the 48 patients, although the majority had skin discomfort.

CONCLUSIONS

Topical prostaglandin E1 gel applied to the penis appears to be safe, and facilitates audiovisual and tactile stimulation resulting in an erection when given in a clinic setting. Consequences to the female partner remain unknown.

Effects of molybdate and pentachlorophenol on the sulfation of alpha-naphthol

Sulfation is the conjugation of chemicals with sulfate, which usually decreases, but occasionally increases, their biological effects. The phenol-sulfotransferase inhibitor pentachlorophenol (PCP) is often used to distinguish the biological effects of a chemical from its sulfate conjugate. Recently, molybdate has been shown to decrease the hepatic concentration of 3'-phosphoadenosine 5'-phosphosulfate (PAPS), the cosubstrate for sulfation. Therefore, the present study was designed to compare the effectiveness and specificity of molybdate and PCP as inhibitors of sulfation. Alpha-naphthol (125 and 250 micromol/kg, i.p.) was administered to rats and the sulfate and glucuronide conjugates excreted into urine were quantified for this comparison. Molybdate (5.0, 7.5, and 10 mmol/kg) decreased the 24-h cumulative urinary excretion of the sulfate conjugate of the lower dose of alpha-naphthol by 54, 53, and 55%, respectively, with corresponding compensatory increases in glucuronide excretion at the two lower doses of molybdate. PCP (20, 40, and 80 micromol/kg) similarly decreased the sulfation of alpha-naphthol by 48, 38, and 41%, respectively, but prevented compensatory increases in glucuronide excretion. Molybdate (2.5, 5.0, and 7.5 mmol/kg) decreased the sulfation of the higher dose of alpha-naphthol by 21, 30, and 44%, respectively, again with corresponding compensatory increases in glucuronide excretion. In contrast, PCP did not decrease significantly the sulfation of the higher dose of alpha-naphthol. These data suggest that molybdate is equally or more effective than PCP at inhibiting sulfation of alpha-naphthol, and appears to be more specific.

Sulfation of minoxidil by multiple human cytosolic sulfotransferases

Minoxidil is an antihypertensive agent and hair growth promoter that is metabolized by sulfation to the active compound, minoxidil sulfate. Thermostable phenol sulfotransferase (TS PST or P-PST) was initially thought to catalyze the reaction, and the enzyme was designated minoxidil sulfotransferase (MNX-ST). Information about human ST activities toward minoxidil would be useful in developing the capacity to predict individual responses to minoxidil based on tissue levels of STs. Therefore, human STs were studied from platelet homogenates, partially purified platelets, scalp skin high speed supernatants and COS-1 cell cDNA expressed preparations using a radiochemical enzymatic assay with minoxidil as the substrate. Studies showed the presence of TS PST, TL (thermolabile) PST and MNX-ST activities in human scalp skin. Biochemical properties and correlation studies suggested that in addition to TS PST, the TL PST activity, another ST activity or both were involved in the reaction. Partially purified human platelet TL PST tested with minoxidil and dopamine showed identical thermal stabilities and similar responses to the inhibitors 2,6-dichloro-4-nitrophenol (DCNP) and NaCl. To characterize the activity of TL PST toward minoxidil, several biochemical properties of the enzyme expressed from a human liver cDNA clone were investigated. When assayed with minoxidil and dopamine, thermal stabilities of the expressed enzyme were identical and IC50 values for the inhibitors DCNP and NaCl were similar. It was also demonstrated that cDNA encoded human liver dehydroepiandrosterone sulfotransferase and estrogen sulfotransferase contributed to the sulfation of minoxidil. The results confirm that at least four human STs contribute to minoxidil sulfation. MNX-ST activity represents a combination of ST activities. The data indicate that multiple ST activities should be taken into account in attempts to predict the regulation of minoxidil sulfation and individual responses to minoxidil.

Photoaffinity labeling of peroxisome proliferator binding proteins in rat hepatocytes; dehydroepiandrosterone sulfate- and bezafibrate-binding proteins

To detect the cellular sites which directly interact with peroxisome proliferators (PPs) and mediate their inducing effect on peroxisomal enzymes in rat hepatocytes, two kinds of radiolabeled ligands, AD12 (7alpha-N-(4-azido-2-hydroxy-5-iodo[125I]benzyl)-aminomethyl-5-and rostene-3beta-ol-17-one-O-3-sulfate) and BZ5 (2-[p-[2-(4'-azido-3',5'-diiodo[125I]benzamido-2'-hydroxy)ethyl]phenoxy] -2-methylpropionic acid), were developed for photoaffinity labeling. These compounds were derivatives of dehydroepiandrosterone sulfate (DHEAS) and bezafibrate, respectively, with an azido group as the photoreactive functional group. Upon UV-irradiation following incubation with rat liver cytosol and nuclei, both the ligands effectively radiolabeled several proteins analyzed by SDS-polyacrylamide gel electrophoresis/radioluminography. When [125I]AD12 was used at a concentration of 0.2 microM, two cytosolic proteins with molecular masses of 55 and 28 kDa and a nuclear protein of 40 kDa were specifically labeled, as coincubation with a 1000-fold excess of DHEAS inhibited labeling. Photoaffinity labeling of the cytosolic 28-kDa protein was also affected by Wy-14,643, but not by unsulfated dehydroepiandrosterone or androsterone sulfate, consistent with our previous findings obtained in competitive binding studies of [3H]DHEAS-binding detected in rat liver cytosol (Yamada et al. (1994) Biochim. Biophys. Acta 1224, 139-146). On the other hand, [125I]BZ5 specifically labeled a cytosolic protein of 31 kDa, which was inhibited by coincubation with bezafibrate, clofibric acid and Wy-14,643, but not with DHEAS. Thus, [125I]AD12 and [125I]BZ5 labeled several proteins which recognized DHEAS and bezafibrate, respectively, in rat liver cytosol and nuclei, providing a useful means to investigate PP-binding proteins.

Characterization of recombinant human liver dehydroepiandrosterone sulfotransferase with minoxidil as the substrate

Biotransformation of xenobiotics and hormones through sulfate conjugation is an important metabolic pathway in humans. The activation of minoxidil, an antihypertensive agent and hair growth stimulator, by sulfation (sulfonation) is carried out by more than one sulfotransferase. Initially only the thermostable form of phenol sulfotransferase was thought to catalyze minoxidil sulfation. We document in this report the new finding that human liver dehydroepiandrosterone sulfotransferase (DHEAST), an hydroxysteroid sulfotransferase distinct from phenol sulfotransferases, also catalyzes the reaction. To characterize more precisely the activity of DHEA ST toward minoxidil, we used COS-1 cells to express DHEA ST from a human liver cDNA clone. The apparent Km values for minoxidil and [35S]3'-phosphoadenosine-5'-phosphosulfate were 3.9 mM and 0.13 microM, respectively. The 50% inactivation temperature of the COS-expressed enzyme was 42 degrees, and the IC50 value for 2,6-dichloro-4-nitrophenol was 1.4 x 10(-4) M. Both the thermal stability behavior and response to DCNP were similar when the cDNA encoded DHEA ST was assayed with DHEA or minoxidil as a substrate. NaCl led to a greater activation of the cDNA expressed DHEA ST when assayed with DHEA (2.5-fold) than when the same preparation was assayed with minoxidil (1.4-fold). These data indicate that DHEA ST catalyzes the sulfate conjugation of minoxidil: DHEA ST activity present in the human gut and liver would be expected to add to the overall sulfate conjugation of orally administered minoxidil. Thus, DHEA ST activity must be considered when determining the human tissue sulfotransferase contribution to minoxidil sulfation.

The pharmacologic basis of the cardiovascular toxicity of minoxidil in the dog

Minoxidil (MNX), like several other vasoactive drugs, causes cardiovascular toxicity in dogs by undetermined mechanisms. We studied the mechanism of cardiovascular toxicity of MNX [an adenosine triphosphate (ATP)-sensitive potassium channel opener] by blocking its pharmacologic effects with glyburide (an ATP-sensitive potassium channel blocker) in groups of 5 female beagle dogs treated orally for 2 days with 1.0 mg/kg/day of MNX alone or with glyburide given in 5 or 6 divided doses of 300 mg/kg at 2 hr before and after each dose of MNX and at 3-6-hr intervals thereafter. A third group of 5 dogs received glyburide alone in the same dosing regimen as in the combination group. Mean arterial pressure (MAP), heart rate (HR), the pharmacokinetics of MNX, and gross and microscopic changes in the heart were evaluated. Glyburide did not influence the pharmacokinetics of MNX but prevented or markedly attenuated the MNX-induced cardiovascular lesions (right atrial hemorrhagic lesions, subendocardial necrosis, or coronary arteritis) occurred in dogs whose MNX-induced hemodynamic effects were effectively blocked by glyburide. In conclusion, the cardiovascular toxicity of MNX in dogs is not caused by a direct toxic effect of MNX on the heart but apparently is related to the exaggerated pharmacologic/profound hemodynamic effects it elicits in the dog.

The effect of minoxidil analogues and metabolites on the contraction of collagen lattices by human skin fibroblasts

Minoxidil, in addition to its effect on hypertension and hair growth, has been proposed as a potential antifibrotic agent. Minoxidil inhibits the contraction of collagen lattices by human fibroblasts in vitro. However, the mechanism of inhibition is unknown. As minoxidil is metabolised in the body to minoxidil glucuronide and minoxidil sulphate, we investigated the potencies of these metabolites to inhibit collagen lattice contraction. We also studied selected analogues of minoxidil to assess the influence of certain functional groups in the inhibition. The major metabolite, minoxidil glucuronide, proved to be inactive, whereas minoxidil sulphate was considerably more active than minoxidil. In terms of the structural analogues, the substitution of one amino group by a methyl group resulted in loss of the inhibitory activity; removal of the nitroxide oxygen led to stronger inhibition than with minoxidil. Further studies are planned to learn more about a possible role for minoxidil in wound contraction.

Pharmacology of the potassium channel openers

The potassium-channel openers comprise a large number of molecules that can be classified into three basic groups: (1) agents like levcromakalim that open a small-conductance (10-30 pS) glibenclamide-sensitive K+ channel currently known as the ATP-sensitive K+ channel, KATP; (2) hybrid molecules, such as nicorandil, that open KATP channels and that also activate the enzyme-soluble guanylate cyclase; (3) molecules like dehydrosaponin 1 that open the large-conductance (100-150 pS) calcium-dependent K+ channel, BKCa. K(+)-channel openers in groups 1 and 2 are most potent on smooth muscle, but KATP channels in cardiac muscle, neurones and the pancreatic beta cell are also affected. In vivo, moderate to high doses produce a fall in diastolic pressure with reflex tachycardia; low doses may exert selective dilator effects on specific vascular beds with little effect on systemic pressure. In vitro, all smooth muscles are relaxed with loss of spontaneous electric and mechanical activity; hyperpolarization to the region of EK is often observed. These effects can be antagonized by glibenclamide and also by imidazolines and guanidines, such as phentolamine, guanethidine, and antazoline, agents that also inhibit the smooth muscle delayed rectifier channel, KV. The mode and site of action of the group 1 and 2 K(+)-channel openers is the subject of intense study. Irrespective of their specific mode of action, the K(+)-channel openers, especially the hybrid molecules such as nicorandil, constitute a novel and promising approach to the treatment of cardiovascular disease.

The same enzymes catalyze sulfation of minoxidil, minoxidil analogs and catecholamines

Rat liver sulfation of minoxidil and minoxidil analogs is described and the enzymes responsible are compared with those that sulfate catecholamines. Our study of minoxidil sulfation showed male-dominant sex dimorphism of the enzyme activity due to two enzymes that coelute with dopamine sulfotransferases. The most abundant isoform, in our routine assay, is minoxidil sulfotransferase 2 (P2). Sulfation ability by this enzyme parallels minoxidil analog antihypertensive ability. Minoxidil, analog and dopamine sulfotransferases were not separable by several different chromatographic methods, supporting their identity. The minoxidil sulfotransferase activity dropped in hypophysectomized males, due mostly to diminished levels of minoxidil sulfotransferase 2/dopamine sulfotransferase II, which appears to be aryl sulfotransferase IV. Its relationship to the minoxidil sulfotransferase reported by the Falany group is not clear. Here, we describe the exploration of rat liver sulfation of antihypertensive minoxidil (2,4-diamino-6-piperidinopyrimidine-3-oxide), and several minoxidil analogs. Minoxidil sulfation was first reported in rat and human liver cytosol by Johnson's group at Upjohn [1,2]. Our interest in the sulfation process arose because direct action of minoxidil sulfate had been implicated both in vivo [3,4] and in vitro [5] in blood pressure control processes.

Topical minoxidil in the treatment of male erectile dysfunction

Topically applied 2% minoxidil solution has been reported to increase diameter, rigidity and arterial flow to the penis. As a result it has been suggested as a possible treatment for erectile dysfunction. A total of 21 patients received 2% minoxidil for treatment of erectile dysfunction with instructions to apply 1 cc of the solution slowly over the glans penis 20 minutes before intercourse. Average patient age was 52.5 years (range 29 to 65 years). The etiology of the impotence was neurogenic in 8 patients, vascular in 7, psychogenic in 4 and other causes in 2. Two patients also had clinical evidence of venous incompetence and 4 were diabetics. One patient with psychogenic impotence noticed improvement in the duration of erection but no increase in rigidity or size after minoxidil application. One patient with impotence after excision of a Peyronie's plaque reported a rigid erection adequate for intercourse after using minoxidil. This patient subsequently was able to achieve erections without using minoxidil. The remaining 19 patients had no improvement in erectile rigidity, or the ability to obtain or maintain an erection. One patient did notice some mild burning on the glans penis after applying the minoxidil. No other side effects were noted in any patient. These results indicate that 2% topical minoxidil solution is not effective when applied to the penis in the treatment of erectile dysfunction. It is possible that a higher concentration, a different delivery medium or a different chemical composition may yield better results.

Minoxidil sulphation in human liver and platelets. A study of interindividual variability

Minoxidil requires to be sulphated to exert its hypotensive effect. We report on interindividual variability in the rate of minoxidil sulphation in 118 specimens of human liver and in platelets obtained from 100 healthy subjects and 100 newborns. The frequency distribution histogram of the hepatic activity of minoxidil sulphotransferase was positively skewed; the mean was 631 pmol.min-1 x mg-1. After logarithmic transformation of the enzyme activity, the frequency distribution histogram became symmetrical and did not significantly deviate from normality. The rate of minoxidil sulphation was not different in platelets from adults (0.74 pmol.min-1 x mg-1) and newborns (1.16 pmol.min-1 x mg-1). The frequency distribution histograms were positively skewed and the results of normal equivalent deviation analysis was compatible with the presence of at least two subgroups of sulphotransferase in liver and platelets. Thus, two phenotypes of sulphotransferase exist in human liver and platelets, and the "extensive sulphator" phenotype contributes to skewing the frequency distribution. In platelets, the percentage of subjects that fall in the two subgroups is different at birth and in adulthood. This can explain the different shape of the frequency distribution in newborn and adult platelets and suggests that platelet minoxidil sulphotransferase undergoes modification after birth.

Sulfation of hypertensive and hypotensive drugs by monkey brain phenol sulfotransferase

The substrate specificity and affinity of two forms of phenol sulfotransferase (PST) from Rhesus macaque brain cortex were studied. Catecholamines, their methylated metabolites (normetanephrine, metanephrine) and methylated precursor, alpha-methylDOPA, were examined as substrates for both the cationic (PST I) and the anionic (PST II) forms of the enzyme. Sulfation of hypertensive drugs (phenylephrine, octopamine, metaraminol), hypotensive drugs (alpha-methylDOPA, minoxidil), and related agents without a free hydroxy group on the benzene ring were also studied. Results indicated that both PST forms sulfated alpha-methylDOPA and minoxidil, but only PST II transferred the sulfate group to catecholamines and most of the adrenergic agents examined.

Minoxidil Sulfotransferase, a Marker of Human Keratinocyte Differentiation

The sulfation of minoxidil is catalyzed by a sulfotransferase activity in a number of tissues including skin. To investigate further the nature of the minoxidil sulfotransferase activity in epithelial tissue and to compare this activity to that of cholesterol sulfotransferase, which has already been shown to be induced during the differentiation of epithelial cells, we cultured normal human epidermal keratinocytes in a keratinocyte growth medium for 4 d, after which the media were replaced with either the same growth media or media with increasing Ca++ concentrations. Cholesterol sulfotransferase, minoxidil sulfotransferase, and transglutaminase were determined during the differentiation of the cells in the three media. Time-activity curves that suggested two different sulfotransferase activities were induced during the differentiation process. U-77581, a competitive inhibitor of minoxidil sulfotransferase activity, inhibited the sulfation of minoxidil sulfotransferase activity in the keratinocyte homogenates, but it did not inhibit the sulfation of cholesterol. These data indicate that at least two sulfotransferase activities are induced during the differentiation of epithelial keratinocytes and minoxidil sulfotransferase is an early marker of that differentiation.

Inhibition of human liver steroid sulfotransferase activities by drugs: a novel mechanism of drug toxicity?

The inhibition of steroid and phenol sulfotransferase activities in human liver cytosol by a wide range of commonly used drugs was studied. Dehydroepiandrosterone (DHEA) and estrone sulfotransferase activities were strongly inhibited by a number of compounds, with IC50 values ranging between 440 pM and 147 microM. For DHEA sulfotransferase, clomiphene, testosterone, danazol and spironolactone were the best inhibitors, with IC50 values less than 5 microM, whereas for estrone sulfotransferase cyclizine, ibuprofen, chlorpheniramine and dimenhydrinate resulted in the strongest inhibition, again with IC50 values of less than 5 microM. The xenobiotic substrate 1-naphthol was refractory to substantial inhibition, with the exception of clomiphene. The majority of the drugs which inhibited steroid ST activities strongly were either synthetic steroids, antisteroidals or were tertiary amine drugs such as tricyclic antidepressants and antihistamines, many of which exhibit adverse side effects manifesting particularly as sexual dysfunction and disruption of hormone action in clinical use. The importance of these findings for our understanding of the molecular basis of adverse drug reactions is discussed.

Structure-activity relationship of minoxidil analogs as inhibitors of lysyl hydroxylase in cultured fibroblasts

The structural features that confer upon minoxidil the ability to suppress lysyl hydroxylase activity in human skin fibroblasts were investigated. Substitution of the amino group in position 2 or 6 of the pyrimidine ring with a methyl group had no significant effect on the inhibitory activity of minoxidil, whereas substitution of both amino groups with methyl groups resulted in a complete loss of inhibitory activity. Together, these observations indicate that only one of the two amino groups ortho to the nitroxide oxygen is essential for the enzyme-suppressing effect of minoxidil. Derivatives of minoxidil formed by hydroxylation at position 3 or 4 of the piperidine ring were as active as the parent compound in suppressing lysyl hydroxylase activity. However, replacement of the piperidinyl group in position 4 of the pyrimidine ring with a pyrrolidinyl, morpholinyl, or N-methylpiperazinyl group resulted in loss of inhibitory activity, demonstrating that the piperidinyl group para to the nitroxide oxygen is essential for the enzyme-suppressing effect of minoxidil. Removing the nitroxide oxygen from position 1 of the pyrimidine ring resulted in a partial loss of the specificity of minoxidil for suppression of lysyl hydroxylase activity. The results indicate that distinct structural elements determine the enzyme-suppressing effect and the antihypertensive effect of minoxidil.

Molecular enzymology of human liver cytosolic sulfotransferases

Sulfoconjugation of drugs and small xenobiotic and endogenous compounds in human tissues is primarily catalysed by cytosolic forms of sulfotransferase. In most instances, sulfate conjugation is associated with a decrease in the biological activity and an increase in the excretion of the sulfated compounds. However, sulfation may also be involved in the bioactivation of both therapeutic drugs and carcinogens. Two isoenzymes of phenol sulfotransferase and one steroid/bile acid sulfotransferase have been isolated from human liver cytosol. Biochemical and immunological characterization of these proteins has provided insights into their structural similarities, tissue localization and possible biological functions.

Localization of minoxidil sulfotransferase in rat liver and the outer root sheath of anagen pelage and vibrissa follicles

The precise biochemical mechanism and site(s) of action by which minoxidil stimulates hair growth are not yet clear. Minoxidil sulfate is the active metabolite of minoxidil, with regard to smooth muscle vasodilation and hair growth. Formation of minoxidil sulfate is catalyzed by specific PAPS-dependent sulfotransferase(s) and minoxidil-sulfating activities have been previously reported to be present in liver and hair follicles. One of these minoxidil-sulfating enzymes has been purified from rat liver (rat minoxidil sulfotransferase, MST) and a rabbit anti-MST antibody has been prepared. Using this anti-MST antibody, we have immunohistochemically localized minoxidil sulfotransferase in the liver and anagen hair follicles from rat. In rat pelage and vibrissa follicles, this enzyme is localized within the cytoplasm of epithelial cells in the lower outer root sheath. Although the immunolocalization of MST might not necessarily correlate with the MST activity known to be present in anagen follicles, the results of this study strongly suggest that the lower outer root sheath of the hair follicle may serve as a site for the sulfation of topically applied minoxidil.

An HPLC-ECD procedure for measuring total phenolsulfotransferase (PST) activity in human liver, platelets and blood

The phenolsulfotransferase (PST) activity in human liver, platelets and blood was measured under saturating concentrations of the conjugating agent, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Conventional PST assays employ PAP35S at suboptimal concentrations. In addition, the sulfate conjugate formed, namely N-acetyldopamine-sulfate (NADA-sulfate) was quantified directly by high-pressure liquid chromatography cum electrochemical detection (HPLC-ECD). NADA, the biogenic amine acceptor used in this study appeared from kinetic data to be a substrate of both the P and M forms of PST when used in micromolar concentration. Two apparent Km values of 4.2 mumol/l and 22.6 mumol/l were observed. In contrast, only one apparent Km value was evident when the assay was carried out in the presence of 2,6-dichloro-4-nitrophenol (DCNP), a selective inhibitor of the P form of PST or after heat treatment under specified conditions which inactivates the M form of PST. Thus measurement of PST activity with NADA as the acceptor substrate permits the determination of total PST activity and a parallel assay with the inclusion of DCNP would distinguish the two variants of PST, both of which appear to be present in all human tissues.

Minoxidil sulfate is the active metabolite that stimulates hair follicles

An important step in understanding minoxidil's mechanism of action on hair follicles was to determine the drug's active form. We used organ-cultured vibrissa follicles to test whether it is minoxidil or its sulfated metabolite, minoxidil sulfate, that stimulates hair growth. Follicles from neonatal mice were cultured with or without drugs and effects were assessed by measuring incorporation of radiolabeled cysteine in hair shafts of the treated follicles. Assays of minoxidil sulfotransferase activity indicated that vibrissae follicles metabolize minoxidil to minoxidil sulfate. Dose-response studies showed that minoxidil sulfate is 14 times more potent than minoxidil in stimulating cysteine incorporation in cultured follicles. Three drugs that block production of intrafollicular minoxidil sulfate were tested for their effects on drug-induced hair growth. Diethylcarbamazine proved to be a noncompetitive inhibitor of sulfotransferase and prevented hair growth stimulation by minoxidil but not by minoxidil sulfate. Inhibiting the formation of intracellular PAPS with chlorate also blocked the action of minoxidil but not of minoxidil sulfate. Acetaminophen, a potent sulfate scavenger blocked cysteine incorporation by minoxidil. It also blocked follicular stimulation by minoxidil sulfate apparently by directly removing the sulfate from the drug. Experiments with U-51,607, a potent minoxidil analog that also forms a sulfated metabolite, showed that its activity was inhibited by both chlorate and diethylcarbamazine. These studies show that sulfation is a critical step for hair-growth effects of minoxidil and that it is the sulfated metabolite that directly affects hair follicles.

Purification and characterization of rat liver minoxidil sulphotransferase

Minoxidil (Mx), a pyrimidine N-oxide, is used therapeutically as an antihypertensive agent and to induce hair growth in patients with male pattern baldness. Mx NO-sulphate has been implicated as the agent active in producing these effects. This paper describes the purification of a unique sulphotransferase (ST) from rat liver cytosol that is capable of catalysing the sulphation of Mx. By using DEAE-Sepharose CL-6B chromatography, hydroxyapatite chromatography and ATP-agarose affinity chromatography, Mx-ST activity was purified 240-fold compared with the activity in cytosol. The purified enzyme was also capable of sulphating p-nitrophenol (PNP) at low concentrations (less than 10 microM). Mx-ST was purified to homogeneity, as evaluated by SDS/PAGE and reverse-phase h.p.l.c. The active form of the enzyme had a molecular mass of 66,000-68,000 Da as estimated by gel exclusion chromatography and a subunit molecular mass of 35,000 Da. The apparent Km values for Mx, 3'-phosphoadenosine 5'-phosphosulphate and PNP were 625 microM, 5.0 microM and 0.5 microM respectively. However, PNP displayed potent substrate inhibition at concentrations above 1.2 microM. Antibodies raised in rabbits to the pure enzyme detected a single band in rat liver cytosol with a subunit molecular mass of 35,000 Da, as determined by immunoblotting. The anti-(rat Mx-ST) antibodies also reacted with the phenol-sulphating form of human liver phenol sulphotransferase, suggesting some structural similarity between these proteins.

Sulfation of minoxidil by human liver phenol sulfotransferase

The N,O-sulfate of minoxidil (Mnx) is the active agent in producing the vasodilation and the hair-growth stimulating responses observed with Mnx treatment. In this report, Mnx sulfation activity was assayed in cytosol prepared from several normal human livers, and Mnx sulfation was shown to correlate significantly with the activity of the phenol-sulfating form of phenol sulfotransferase (P-PST) activity in the same livers. No correlation was observed between Mnx sulfation and the dopamine or dehydroepiandrosterone (DHEA) sulfotransferase activities present in human liver. Mnx sulfation also copurified with P-PST activity during the purification of P-PST from human liver. During the purification procedure, Mnx and p-nitrophenol sulfotransferase (P-PST) activities were resolved from the dopamine and DHEA sulfation activities catalyzed by the monoamine-sulfating form of phenol sulfotransferase (M-PST) and DHEA sulfotransferase respectively. Also, purified DHEA sulfotransferase was not capable of sulfating Mnx, and no data were obtained to indicate that Mnx is a substrate for M-PST. p-Nitrophenol, a substrate for P-PST, was demonstrated to be a competitive inhibitor of Mnx sulfation catalyzed by purified P-PST when Mnx was the variable substrate. These results indicate that Mnx is sulfated and, therefore, bioactivated by P-PST in human liver.

Potassium channel openers and vascular smooth muscle relaxation

Potassium channel openers comprise a diverse group of chemical agents which open plasma-lemmal K-channels. They show selectivity for smooth muscle, although K-channels in cardiac and skeletal muscle, neurones and the pancreatic beta-cell are also affected at relatively high concentrations. In addition, at least one endogenous K-channel opener of vascular origin--endothelium-derived hyperpolarizing factor--exists and in man plays a role in modulating blood vessel tone. The type of K-channel involved in the actions of both exogenous and endogenous K-channel openers is still uncertain, although a prime candidate in smooth muscle seems similar to the [ATPi]-modulated K-channel in the pancreatic beta-cell. This review focuses attention on the action of these agents in vascular smooth muscle and on the possible clinical exploitation of their powerful vasorelaxant properties.

Sulfation of minoxidil by human platelet sulfotransferase

In an attempt to determine whether (1) sulfotransferase activity in human platelets would convert minoxidil to minoxidil sulfate and (2) inter-subject variations in this sulfotransferase activity could be noted, platelet homogenates were incubated with minoxidil and 35S-PAPS in HEPES buffer at 37 degrees C for 30 min. Radioactivity which was extracted into ethyl acetate and shown by HPLC to elute with authentic minoxidil sulfate was counted by scintillation counting. Aliquots of the platelet homogenates were also preincubated at 43 degrees C for 15 min to determine the thermal stability of the sulfotransferase activity. Sulfotransferase activity in platelets from 48 adult males ranged from 0.9-13.2 pmol minoxidil sulfate produced/10(7) platelets per 30 min (mean 4.91 +/- 2.84 pmol/10(7) platelets per 30 min +/- SD). Thermal stable sulfotransferase activity ranged from 0.2-7.6 pmol minoxidil produced/10(7) platelets per 30 min and varied from 15 to 57% of the total sulfotransferase activity. Thus, the results indicate that human platelets can effect the sulfation of minoxidil and that sulfotransferase activity does show inter-subject variation.

Action of topical minoxidil in the bald stump-tailed macaque

The effect of topical minoxidil (5% and 2% solutions) on hair regrowth was studied in the frontal bald scalp of 18 adolescent and adult stump-tailed macaques (Macaca arctoides). Gross observation of the hairiness and folliculogram analysis of the skin biopsy specimens have shown that minoxidil induces the enlargement of vellus follicles to the size of middle to terminal follicles (regrowth of hair effect), minoxidil maintains the terminal follicles in the prebald scalp of periadolescent animals (prevention of baldness effect), enlarged follicles regress after minoxidil is withdrawn, and hair follicular growth is once again stimulated when treatment with minoxidil is reinstituted. Hair regrowth was more prominent in the early stage of baldness among younger macaques than in baldness of longer duration in older animals. An in vitro study of 3H thymidine uptake revealed that the hair follicles in minoxidil-treated macaque skin showed significant enhancement of deoxyribonucleic acid synthesis in the follicular and perifollicular cells but not in the epidermal keratinocytes. Furthermore, the uptake of 3H minoxidil and its conversion to minoxidil sulfate (the active metabolite producing vasodilation) was relatively higher in the hair follicles than in the epidermis and dermis. Serum concentration of minoxidil was fairly constant 2, 4, 6, 15, and 24 hours after a single application (averaging 15 ng/ml with 5% minoxidil). Minoxidil's essential action in hair follicular growth may be as a potent vasodilator. However, a direct action on the hair follicle cannot be ruled out considering uptake and conversion of the drug to minoxidil sulfate within the hair follicle itself.

Minoxidil analysis in human plasma using high-performance liquid chromatography with electrochemical detection. Application to pharmacokinetic studies

A method is described for determination of minoxidil in human plasma using high-performance liquid chromatography with electrochemical detection. The method is specific and sensitive (500 pg/ml), however, minoxidil and minoxidil sulfate cannot be differentiated due to rapid autohydrolysis of minoxidil sulfate to minoxidil. The extraction procedure employs a C18 preparatory column to remove endogenous plasma constituents which would interfere with the assays. The calibration curves are linear for concentrations from 500 pg to 10 ng/ml. Within-day and between-day reproducibility are satisfactory with coefficient of variation less than 5.7% for all concentrations. Sample recovery from extraction is consistent at 45 to 55% at low and high concentrations, respectively. A pharmacokinetic study in a hypertensive volunteer receiving two different oral doses of minoxidil (1.25 and 2.5 mg) on different occasions demonstrates the utility of the method.