A new, long-lasting competitive inhibitor of angiotensin.

An analog of angiotensin II, [Sar(1), Ile(8)]-angiotensin II, has a potent and long-lasting competitive antagonistic effect against angiotensin II when tested for its myotropic action on the isolated rabbit aorta and for its effect on blood pressure in anesthetized cats and dogs. Compared to [Ile(8)]-angiotensin II, the new analog has equal antagonistic potency on the isolated system but a much greater potency in vivo. It is assumed that sarcosine in position 1 protects the peptide against enzymatic degradation and enhances its half-life. This study demonstrates that the modification in both positions 1 and 8 are important for the in vivo antagonistic potencies of angiotensin analogs.

Losartan and low-dose hydrochlorothiazide in patients with essential hypertension. A double-blind, placebo-controlled trial of concomitant administration compared with individual components.

BACKGROUND: Angiotensin II acts at the cellular level through specific angiotensin II subtype I, AT-1 receptors. Losartan is the first of a new class of antihypertensive agents that specifically block angiotensin II at AT-1 receptors. By acting on complementary and different pharmacologic mechanisms, the concomitant use of low doses of hydrochlorothiazide with losartan may offer an additive antihypertensive activity with fewer adverse experiences. METHODS: This double-blind study evaluated losartan concomitantly administered with hydrochlorothiazide as initial therapy in 703 patients with essential hypertension. RESULTS: The greatest reduction in blood pressure was observed in the 50 mg of losartan potassium and 12.5 mg of hydrochlorothiazide group (17.2 mm Hg in sitting systolic blood pressure and 13.2 mm Hg in sitting diastolic blood pressure [P < or = .001]), and the effects of the two components appeared to be additive. Seventy-eight percent of the patients treated with 50 mg of losartan potassium and 12.5 mg of hydrochlorothiazide had an excellent or good antihypertensive response (sitting diastolic blood pressure < 90 mm Hg or > or = 90 mm Hg with a reduction of 10 mm Hg or more). Peak (6 hours after dosing) and trough placebo-adjusted ratios for the losartan-hydrochlorothiazide groups ranged from 62% to 85%, indicating that there was a smooth reduction in sitting diastolic blood pressure that was sustained over 24 hours. The most common clinical adverse experiences (> or = 4%) that occurred with an incidence slightly greater than that reported by the placebo-treated patients were headache, asthenia or fatigue, dizziness, sinusitis, and upper respiratory infection. CONCLUSION: The concomitant administration of losartan potassium, 50 mg, with 12.5 mg of hydrochlorothiazide once daily produced an additive reduction in trough sitting systolic and diastolic blood pressure and was well tolerated.

Cardiac and noncardiac atrial natriuretic factor (ANF) biosynthesis and storage and plasma ANF in the rat model of chronic ventricular dysfunction.

To evaluate the role of atrial natriuretic factor (ANF) in chronic heart failure (HF), the biosynthesis and storage of ANF in cardiac and noncardiac tissues and the level of plasma ANF were measured in rats exhibiting minimal [2-fold rise in left ventricular end diastolic pressure; myocardial infarct (MI) scar length, 25% left ventricle (LV)] and moderate-severe (3-fold rise in left ventricular end diastolic pressure; decreased contractility (dp/dtmax); MI scar length, 47% LV) chronic HF 30 and 60 days after coronary arterial ligation. In rats with moderate-severe HF (30 days post-MI), the cardiac ANF mRNA concentration (determined by dot blot analysis) increased in three heart chambers [LV, 6-fold; left atria (LA), 3-fold; right ventricle (RV), 2-fold], cardiac immunoreactive ANF (IRANF; determined by RIA) concentration increased on the left side (LV, 7-fold; LA, 33%), but was unchanged (RV) or reduced on the right side (right atria, 33%), and plasma IRANF increased 3-fold above sham control values. Excluding the LV (used for MI scar length), the pattern and magnitude of change in ANF mRNA concentration in moderate-severe HF at 60 days were similar to those at 30 days; the cardiac IRANF concentration at this time was the same (LA) or less than (RV, 66%) sham values, and plasma IRANF increased 6-fold above respective sham values. Generally, the changes in the concentrations of cardiac ANF message and peptide and levels of circulating ANF peptide were smaller in rats with minimal HF. The minute quantities of ANF mRNA and IRANF detected in noncardiac tissues (lung, liver, pituitary, aortic arch, brain, kidney, and salivary gland) were unaltered by HF. These findings show that chronic HF, as defined by hemodynamic and histological measurements, specifically and continuously stimulates atrial as well as ventricular ANF biosynthesis; levels of plasma and cardiac ANF are increased early in HF, but with time are subject to modulation. The cardiac ANF system is the prime locus for the effects of HF, as noncardiac ANF biosynthesis and storage are undisturbed by chronic HF.

New centrally acting antihypertensive drugs related to methyldopa and clonidine.

It has been well established that the antihypertensive drugs clonidine and methyldopa lower blood pressure by acting on postsynaptic alpha 2-adrenergic receptors within cardiovascular control centers of the brain. A number of novel agents designed as lipophilic and highly selective alpha 2-adrenergic stimulants have been synthesized and in general the pharmacological features of these agents resemble clonidine or alpha-methylnorepinephrine, the principal metabolite of methyldopa. The clonidine analogs, ICI-106,270, UK-14,304, piclonidine (LR-99,853), and the bridge analogs (ST-1913, ST-1966,ST-1967) exhibit varying activity on the central cardiovascular control centers. ICI-106,270 is of interest because relative to clonidine it appears to exert fewer CNS side effects. Azepexole (BHT-933) is also of interest because, although structurally unrelated to clonidine, it appears to interact with central alpha-adrenergic receptors in a manner similar to that of clonidine. In contrast, central administration of ST-1966, a monoatomic bridge analog of clonidine, lowers blood pressure in animals treated with an alpha 2-antagonist, which suggests other mechanisms may be involved in its action. Novel antihypertensive agents structurally similar to methyldopa have not been described, although viable pro-drugs of methyldopa such as 2-oxo-1,3-dioxol-4-yl-methyl and pivaloyloxyethyl esters have been shown to have greater oral activity than methyldopa, presumably because they are more lipophilic than the parent moiety.(ABSTRACT TRUNCATED AT 250 WORDS)

Three candidate genes and angiotensin-converting enzyme inhibitor-related cough: a pharmacogenetic analysis.

Unexplained, persistent cough limits the use of angiotensin-converting enzyme (ACE) inhibitors in a significant number of patients. It has been speculated that occurrence of this adverse effect is genetically predetermined; in particular, variants of the genes encoding ACE, chymase, and B2-bradykinin receptor have been implicated. To investigate this question, we determined genotypes for common polymorphisms for these three genes in subjects with a history of ACE inhibitor-related cough. Specificity of the adverse effect was confirmed by a blinded, double-crossover design protocol in which subjects were rechallenged with either lisinopril or placebo. In 99 case subjects and 70 control subjects (who failed to develop cough on rechallenge with ACE inhibitor) thus selected, frequencies for the ACE D and I alleles were 0.56 and 0.44 (cases) and 0.56 and 0.44 (controls), respectively; frequencies for chymase A and B alleles (absence/presence of BstXI site) were 0.56 and 0.44 (cases) and 0.46 and 0.54 (controls), respectively; frequencies for B2-bradykinin receptor + and - alleles (presence/absence of a 21 to 29 nonanucleotide sequence) were 0.52 and 0.48 (cases) and 0.53 and 0.47 (controls), respectively. All observed genotype frequencies were in Hardy-Weinberg equilibrium. There was no evidence for association between genotype at either gene examined and cough (adjusted for gender and age). Our data indicate that common genetic variants of ACE, chymase, and B2-bradykinin receptor do not explain the occurrence of ACE inhibitor-related cough.

Effects of losartan on a background of hydrochlorothiazide in patients with hypertension.

The purpose of this multicenter trial was to compare the antihypertensive efficacy and safety of losartan potassium (losartan), a selective angiotensin II receptor antagonist, when added to hydrochlorothiazide in hypertensive patients whose blood pressure was not adequately controlled by 25 mg hydrochlorothiazide monotherapy. After a 4-week monotherapy period of 25 mg hydrochlorothiazide, 304 patients with trough (22 to 26 hours postdose) sitting diastolic pressure between 93 and 120 mm Hg were maintained on 25 mg hydrochlorothiazide and randomized double-blind into treatment arms consisting of either 25, 50, or 100 mg losartan or placebo once daily for 12 weeks. The reductions in sitting diastolic pressure for patients treated with 25, 50, or 100 mg losartan concomitantly administered with 25 mg hydrochlorothiazide were significantly greater (P < or = .05) than the reductions observed in the 25 mg hydrochlorothiazide plus placebo group beginning 1 week after randomization. The antihypertensive response in all groups was greater at week 3 than week 1, with some additional decrease in blood pressure in some groups at later times. Sitting systolic pressures were also significantly reduced in each group over time. Standing blood pressures at week 12 were similar to sitting blood pressures. A dose-response relationship to losartan was observed in this patient population. The percentages of the total drug-related clinical adverse experiences as assessed by the investigator were generally similar in the 25, 50, and 100 mg losartan plus 25 mg hydrochlorothiazide groups (10.3%, 24.4%, and 20.0%, respectively) compared with the placebo plus 25 mg hydrochlorothiazide group (24.7%).(ABSTRACT TRUNCATED AT 250 WORDS)

A randomized, placebo-controlled, double-blind, parallel study of various doses of losartan potassium compared with enalapril maleate in patients with essential hypertension.

The efficacy and safety of various doses of losartan potassium, a specific and selective angiotensin II receptor antagonist, were compared with those of placebo and enalapril maleate 20 mg in patients with mild to moderate essential hypertension in a randomized, double-blind, parallel study. We randomly allocated 576 patients at the end of a 4-week placebo baseline period to 8 weeks of once-daily double-blind treatment with losartan potassium 10, 25, 50, 100, or 150 mg, enalapril maleate 20 mg, or placebo. After 8 weeks of treatment, mean reductions from baseline in supine systolic/diastolic pressure 24 hours after dosing (trough) for losartan potassium 10, 25, 50, 100, and 150 mg, enalapril maleate 20 mg, and placebo were 7.6/7.9, 7.8/6.8, 13.0/10.1, 8.9/9.9, 10.5/9.7, 14.7/11.2, and 3.8/5.6 mm Hg, respectively. Compared with mean changes in supine diastolic pressure in the placebo group, losartan potassium 50 to 150 mg and enalapril maleate 20 mg produced clinically important and statistically significant reductions (P < or = .01) in blood pressure. At 24 hours after dosing, the blood pressure changes obtained with losartan potassium 50 mg were essentially identical to those obtained with enalapril maleate 20 mg. While there was a dose-related effect with losartan potassium from 10 to 50 mg at peak (6 hours after dosing), doses of 10 and 25 mg were not consistently different from placebo 24 hours after dosing. To assess the once-daily effect of losartan potassium, trough-to-peak ratios of the mean changes in supine diastolic pressure after 8 weeks of treatment were calculated.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of renin and converting enzyme inhibition in sodium-deficient dogs.

While renin is a highly specific protease, converting enzyme has at least two principal substrates, angiotensin I and bradykinin. Changes in the rate of formation of angiotensin II or degradation of bradykinin can influence the hypotensive action of angiotensin converting enzyme inhibitors. The present study was designed to determine if there were differences in the maximal blood pressure reduction in Na-deficient dogs after angiotensin converting enzyme or renin inhibitor treatment. Five conscious dogs received 0.1, 0.5, and 1.0 mg/kg of i.v. enalaprilat, a potent angiotensin converting enzyme inhibitor, which reduced blood pressure to 75 +/- 4, 71 +/- 5, and 71 +/- 5 mm Hg. Plasma immunoreactive angiotensin II levels were reduced in a dose-related fashion to 35% of control level at the highest dose. Infusion of a maximally effective dose of a statine-containing renin inhibitor (SCRIP) with the high dose of enalaprilat produced no further fall in blood pressure (68 +/- 7 mm Hg), but immunoreactive angiotensin II levels fell to essentially zero in four of five dogs. The order of drug administration was reversed in another experiment in a group of nine dogs in which SCRIP reduced plasma immunoreactive angiotensin II to 25% of control at 0.04 mg/kg/minute (n = 5), with reduction to near zero levels at higher doses. Maximal blood pressure reduction was achieved at 0.32 to 0.64 mg/kg/minute (76 +/- 4 mm Hg); 1 mg/kg of enalaprilat lowered blood pressure an additional 11 +/- 2 mm Hg (p less than 0.01) while not further decreasing immunoreactive angiotensin II levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic effects of angiotensin and renin inhibition in dogs with acute left ventricular failure.

The effects of enalaprilat (MK-422), an angiotensin converting enzyme inhibitor, were compared to those of SCRIP, a renin inhibitor, in experimentally induced left ventricular failure. In anesthetized dogs, acute left ventricular failure was induced by repeated embolization, via the left main coronary artery, with 50 microns plastic microspheres. Embolization significantly increased left ventricular enddiastolic pressure from 6 +/- 1 to 14 +/- 1 (p less than 0.05) mm Hg and decreased both left ventricular maximal dP/dt (3,135 +/- 338 to 1,636 +/- 126 mm Hg/second, p less than 0.05) and cardiac output (3.0 +/- 0.3 to 1.6 +/- 0.1 liters per minute, p less than 0.05). Embolization also significantly reduced heart rate and mean arterial pressure. These parameters remained stable after induction of heart failure. Forty-five minutes after embolization, 16 dogs received enalaprilat (100 microns/kg intravenously) and six dogs received SCRIP (100 microns/kg intravenously followed by 10 microns/kg per minute). Both agents caused similar reductions in left ventricular end-diastolic pressure (21 percent versus 26 percent) and total peripheral resistance (25 percent versus 32 percent) and rise in peak positive cardiac contractility, as measured by (dP/dt)/P, (12 percent versus 11 percent). The data suggest that inhibition of angiotensin II formation by two agents, each or which acts at a different point in the cascade, results in similar beneficial hemodynamic effects in dogs with acute left ventricular failure. In addition, angiotensin converting enzyme inhibition failed to further increase sodium excretion and glomerular filtration rate caused by embolization. In summary, inhibition of angiotensin II production by two different inhibitors of the renin system causes an improvement in left ventricular performance in a model of acute experimental left ventricular failure.

Comparative effects of combination drug therapy regimens commencing with either losartan potassium, an angiotensin II receptor antagonist, or enalapril maleate for the treatment of severe hypertension.

OBJECTIVE: To compare the efficacy and safety of a regimen of losartan potassium (losartan) and a regimen of enalapril maleate (enalapril) in a randomized trial of patients with severe hypertension in which the initial treatments were blinded. DESIGN AND METHODS: Seventy-five patients, 23-74 years of age, with sitting diastolic blood pressure of 115-130mmHg, were enrolled in a 12-site multicenter study. The primary efficacy parameters were the change in trough systolic and diastolic blood pressure, as well as response to treatment in terms of categories of hypertensive response. RESULTS: A gradual reduction in mean sitting diastolic blood pressure was observed in all patients treated from week 1 to 12 (10-29mmHg for the losartan regimen and 14-32 mmHg for the enalapril regimen). At week 4, a substantial number of patients remained on monotherapy at either the initial dose or double the dose of losartan (52%) or enalapril (72%). The blood pressure curves for each treatment were parallel over time. The enalapril-based regimen elicited a statistically significantly greater reduction in blood pressure than the losartan-based regimen, although the mean differences in the blood pressure response between the two treatment groups was small. Based on sitting diastolic blood pressure < 90 mmHg or a reduction in blood pressure of at least 10 mmHg, 98% of the patients assigned to the losartan regimen and 100% of the patients assigned to the enalapril regimen had a satisfactory response with a regimen of one to three antihypertensive drugs. Headache was the most common adverse experience in both treatment groups (occurring in 22% of patients assigned to the losartan regimen and 20% of patients assigned to the enalapril regimen). CONCLUSIONS: In this study, the losartan-based regimen effectively lowered blood pressure, was generally well tolerated, and was generally similar to the enalapril-based regimen in the treatment of patients with severe hypertension.

Approaches to vasodilating/beta-adrenergic blocking agents: examples of the dihydrolutidine type.

The aminohydroxypropoxy moiety has been incorporated into the dihydrolutidine class of vasodilators. In the spontaneously hypertensive rat, one of these, (S)-4-[2-methyl-4-[3-(tert-butylamino)-2-hydroxypropoxy]phenyl]-3,5-dicarboethoxy-1,4-dihydrolutidine (4c), exhibited antihypertensive activity on the order of the standard 4-[2-(trifluoromethyl)phenyl]-3,5-dicarboethoxy-1,4-dihydrolutidine (2a). This antihypertensive activity could not be explained in terms of a vasodilating effect, as determined in the dog. In this latter model, 2a decreased both mean arterial and hindlimb perfusion pressures.

Heterocyclic analogues of the antihypertensive beta-adrenergic blocking agent (S)-2-[3-(ter-butylamino)-2-hydroxypropoxy]-3-cyanopyridine.

The synthesis of a series of isoelectronic analogues of (S)-2-[3-(tert-butylamino)-2-hydroxypropoxy]-3-cyanopyridine (1) are described; included in this group are examples of thiazole, isothiazole, thiadiazole, pyrazine, and the structurally related naphthyridines. All of the compounds are similar to 1 in that they contain a cyano group ortho to the aminohydroxypropoxy side chain and meta to the nitrogen heteroatom. In addition, several related examples, having additional nuclear substituents and/or groups other than CN in the position adjacent to the aminohydroxypropoxy group, were prepared, and beta-adrenoceptor antagonist activity and vasodilating potency were determined. Three compounds, thiazole 2 and isothiazoles 3 and 27, effectively lowered mean arterial pressure in the SH rat at 5 mg/kg. Compounds 2, 3, and 27 increased iliac blood flow and exhibited beta-adrenergic blocking properties in the dog.

2-(Aminomethyl)phenols, a new class of saluretic agents. 2. Synthesis and pharmacological properties of the 5-aza isostere of 2-(aminomethyl)-4-(1,1-dimethylethyl)-6-iodophenol.

The synthesis and biological evaluation of 4-(aminomethyl)-6-(1,1-dimethylethyl)-2-iodo-3-pyridinol dihydrochloride (7b) are described. Compound 7b proved to be highly active as a saluretic diuretic in both rats and dogs.

Beta 1-selective adrenoceptor antagonists: examples of the 2-[4-[3-(substituted-amino)-2-hydroxypropoxy]phenyl]imidazole class.

A series of 2-[4-[3-(substituted-amino)-2-hydroxypropoxy]phenyl]imidazoles is described. The compounds were investigated in vitro for beta-adrenoceptor antagonism, and several examples were found to be selective for the beta 1-adrenoceptor. The structure--activity relationship exhibited by this series of compounds is discussed. (S)-2-[p-[3-[[2-(3,4-dimethoxyphenyl)ethyl]amino]-2-hydroxypropoxy]phenyl]-4-(2 -thienyl)imidazole [(S)-13] was over 100 times more selective than atenolol for the beta 1-adrenergic receptor and has been selected for in-depth studies.

An approach to peripheral vasodilator-beta-adrenergic blocking agents.

The syntheses of 2-phenyl- and 2-pyridyl-4-trifluoromethylimidazoles having a 3-tert-butylamino-2-hydroxypropoxy moiety attached to the aryl or heteroaryl substituent are described. Structure--activity relationships based on results from an evaluation of these compounds for antihypertensive, vasodilating, and beta-adrenergic blocking activities are discussed.

3-Hydroxy-alpha-methyltyrosine progenitors: synthesis and evaluation of some (2-oxo-1,3-dioxol-4-yl)methyl esters.

The (5-methyl-2-oxo-1,3- dioxol -4-yl)methyl and (5-tert-butyl-2-oxo-1, 3- dioxol -4-yl)methyl esters of 3-hydroxy-alpha-methyltyrosine (methyldopa) were prepared and evaluated as progenitors of the amino acid. 1H NMR experiments reveal that the esters are converted cleanly to methyldopa and the corresponding alpha-diketone at pH 7.4, with the 5-methyl derivative undergoing hydrolysis faster than the 5-tert-butyl analogue. Bioavailability studies in dogs show that the esters, particularly the 5-methyl derivative, yield significant plasma levels of methyldopa. Both esters are orally effective antihypertensive agents in spontaneously hypertensive (SH) rats. These studies indicate that (2-oxo-1,3- dioxol -4-yl)methyl esters are viable prodrugs for the latentiation of methyldopa.

2-(Aminomethyl)phenols, a new class of saluretic agents. 6. Effects of N,O-spiroannulation and subsequent quaternization.

The synthesis of a number of 3,4-dihydrospiro-2H-1,3-benzoxazines and their corresponding benzoxazinium salts are reported. The saluretic effects displayed by these N,O-spiroannulated 2-(aminomethyl)phenols appear to be, in part, inversely related to their respective in vivo rates of hydrolysis. Good antihypertensive effects are found only in spirobenzoxazinium 22. Thus, a combination of spiroannulation and quaternization on 2 to produce 22 leads to a loss of saluretic effects with maintenance of antihypertensive effects and, thereby, serves to separate these pharmacological properties.

Beta 1-selective adrenoceptor antagonists: examples of the 2-[4-[3-(substituted amino)-2-hydroxypropoxy]phenyl]imidazole class. 2.

An attempt to develop a highly cardioselective beta-adrenoceptor antagonist devoid of intrinsic sympathomimetic activity (ISA) focused on exploring structure-activity relationships around (S)-[p-[3-[[2-(3,4-dimethoxyphenyl)ethyl]amino]-2-hydroxypropoxy] phenyl]-4-(2-thienyl)imidazole. Strategies to reduce or eliminate ISA centered on structural changes that could influence activation of the receptor by the drug itself or by a metabolite. The approaches involved (a) eliminating the acidic imidazole N-H proton, (b) incorporating substituents ortho to the beta-adrenergic blocking side chain, (c) increasing steric bulk around the N-H moiety, (d) decreasing lipophilicity, (e) introducing intramolecular hydrogen bonding involving the imidazole N-H, and (f) displacing the imidazole ring from an activating position by the incorporation of a spacer element. The compounds were investigated in vitro for beta-adrenoceptor antagonism and in vivo for ISA. From these studies, the most successful variation involved the insertion of a spacer between the imidazole and aryl rings. (S)-4-Acetyl-2-[[4-[3-[[2-(3, 4-dimethoxyphenyl)ethyl]amino]-2-hydroxypropoxy]phenyl]methyl] imidazole (S-51) was demonstrated to be highly cardioselective (dose ratio beta 2/beta 1 greater than 9333) and devoid of ISA.

beta-Adrenergic blocking agents with acute antihypertensive activity.

Modification of the pharmacological profile of the vasodilating/beta-adrenergic blocking agent 2-[4-[3-(tert-butylamino)-2-hydroxypropoxy]phenyl]-4-(trifluoromethyl)imidazole (1) has been investigated. Introduction of selected substitutents onto the imidazole ring, in place of the trifluoromethyl group, has yielded highly cardioselective beta-adrenergic blocking agents such as 7, 17, and 18. The placement of alkyl or chloro groups onto the aryl ring of 1, as illustrated by 33, has produced a class of compounds characterized as antihypertensive beta-adrenergic blocking agents. In these examples, the acute antihypertensive activity does not appear to be due to either vasodilating or beta 2-agonist properties.

Antihypertensive beta-adrenergic blocking agents: N-aralkyl analogues of 2-[3-(tert-butylamino)-2-hydroxypropoxy]-3-cyanopyridine.

An interest in dual-acting antihypertensive agents, specifically those related to (S)-2-[3-(tert-butylamino)-2-hydroxypropoxy]-3-cyanopyridine (1), led us to probe the contribution of the side-chain amino substituent in this series. The ability of 1 and its various analogues to displace radiolabeled alpha 1 (WB-4101 and prazosin) and beta (dihydroalprenolol) adrenergic receptor ligands was assessed by receptor-binding techniques. Most of the compounds exhibited high beta-adrenoceptor binding affinities, but only the N-aralkylamino-substituted compounds showed high alpha 1-adrenoceptor affinities. Therefore, the vasodilation shown by 1 was not due to an interaction with the alpha 1 adrenoceptor. The aralkylamino analogues of 1 in spontaneously hypertensive rats and anesthetized dogs exhibited antihypertensive activity and alpha 1-adrenoceptor blocking properties. Unlike the preference shown by beta-adrenoceptors for S enantiomers in this oxymethylene class of beta blockers, the chirality at the secondary hydroxy center made only a minor contribution to the affinity for the alpha 1-adrenoceptor and even less of a contribution to the observed antihypertensive effects. This lack of chiral influence at the hydroxy center confirmed what had been previously observed in more limited studies with the isomers of both labetalol and medroxalol.