Blood-brain transfer and antinociception of linear and cyclic N-methyl-guanidine and thiourea-enkephalins.

Enkephalins are active in regulation of nociception in the body and are key in development of new synthetic peptide analogs that target centrally located opioid receptors. In this study, we investigated the in vivo blood-brain barrier (BBB) penetration behavior and antinociceptive activity of two cyclic enkephalin analogs with a thiourea (CycS) or a N-methyl-guanidine bridge (CycNMe), and their linear counterparts (LinS and LinNMe) in mice, as well as their in vitro metabolic stability. (125)I-LinS had the highest blood-brain clearance (K1=3.46µL/gmin), followed by (125)I-LinNMe, (125)I-CycNMe, and (125)I-CycS (K1=1.64, 0.31, and 0.11µL/gmin, respectively). Also, these peptides had a high metabolic stability (t1/2>1h) in mouse serum and brain homogenate, and half-inhibition constant (Ki) values in the nanomolar range with predominantly µ-opioid receptor selectivity. The positively charged NMe-enkephalins showed a higher antinociceptive activity (LinNMe: 298% and CycNMe: 205%), expressed as molar-dose normalized area under the curve (AUC) relative to morphine, than the neutral S-enkephalins (CycS: 122% and LinS: 130%).

Effects of 1,1-dimethylguanidine administration on blood pressure, heart rate and renal sympathetic nerve activity in normotensive and spontaneously hypertensive rats.

1,1-dimethylguanidine (DMG) is an endogenous nitric oxide (NO) synthesis inhibitor. This study investigates the effects of exogenous DMG administration, in anaesthetized spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Mean blood pressure (MBP), heart rate (HR) and renal sympathetic nerve activity (SNA) were recorded in 12- to 14-week old, anaesthetized SHR and WKY. Each rat received increasing bolus injections of DMG intravenously (1.03, 2.05, 6.39, 20.45 and 51.15 mg kg-1). In separate experiments, SHR received L-arginine or D-arginine in a dose of 300 mg kg-1 followed by DMG at 6.39 mg kg-1. Thirty minutes later they received the same doses of the respective arginines followed by DMG at 20.45 mg kg-1 DMG induced dose-dependant increases in MBP in SHR and WKY. In SHR, HR increased with increasing doses of DMG (except at the near-toxic doses of 51.15 mg kg-1), whereas in WKY HR decreased with increasing doses of DMG. The net change of renal SNA ranged from -5 +/- 3 to -55 +/- 12% in SHR and from -6 +/- 8 to -66 +/- 8% in WKY. Pre-treatment with L-arginine in SHR partly inhibited the pressor effect and attenuated the inhibition of renal SNA induced by DMG, but had little effect on HR. Thus the administration of NO inhibitor DMG could alter cardiovascular function and sympathetic nerve activity, and subsequently resulted in tachycardia in SHR and bradycardia in WKY.

Effect of methylguanidine, guanidine and structurally related compounds on constitutive and inducible nitric oxide synthase activity.

The inhibitory activity of methylguanidine, guanidine and their precursors, creatine and creatinine, on both the neuronal constitutive and lung inducible isoforms of nitric oxide synthase were examined in this study. Methylguanidine and guanidine (0.01-3 mM) significantly (P<0.01) inhibited in a concentration-dependent manner both isoforms of the enzyme. Furthermore analysis of the inhibition curves by ANOVA revealed that methylguanidine and guanidine act as non selective inhibitors of both nitric oxide synthases (P>0.4 for both methylguanidine and guanidine). In contrast, creatine and creatinine, although containing guanidine group, were totally ineffective on either enzyme even at concentration up to 3 mM. The results obtained for tested compounds also suggest a role for the lateral chain of guanidine group in the enzyme inhibition. The lack of selectivity of methylguanidine and guanidine in inhibiting both the nitric oxide synthase enzymes could account for some pathological manifestations like neurological disorders, host defense impairment and probably hypertension, that often occur in patients with uremia or chronic renal failure.

Relative contributions of advanced glycation and nitric oxide synthase inhibition to aminoguanidine-mediated renoprotection in diabetic rats.

Advanced glycation end products (AGEs) have previously been shown to be increased in the diabetic kidney. Aminoguanidine, an inhibitor of advanced glycation, has been shown to attenuate the development of AGEs as well as the progression of renal disease in experimental diabetes. However, the precise mechanisms through which aminoguanidine acts remain to be elucidated since it is also able to act as an inhibitor of nitric oxide synthase (NOS). This study has therefore compared the effects of aminoguanidine with the effects of two other inhibitors of NOS, L-NAME and methylguanidine, on the development of experimental diabetic nephropathy. Diabetic rats were randomised to receive no treatment, aminoguanidine (1 g/l in drinking water), L-NAME (5 mg/l in drinking water) or methylguanidine (1 g/l in drinking water). Diabetic rats had increased levels of albuminuria and urinary nitrite/nitrate excretion when compared to control rats. Renal AGEs measured by fluorescence as well as by a carboxymethyllysine reactive radioimmunoassay, were elevated in diabetic rats. No changes in inducible NOS (iNOS) protein expression were detected in experimental diabetes nor did aminoguanidine affect iNOS expression. Aminoguanidine did not affect blood glucose or HbA1c but it did prevent increases in albuminuria, urinary nitrites/nitrates and renal AGE levels as measured by fluorescence and radioimmunoassay. L-NAME and methylguanidine did not retard the development of albuminuria, nor did they prevent increases in renal AGE levels, as assessed by fluorescence. However, these treatments did prevent increases in AGEs, as measured by radioimmunoassay. This study indicates that the renoprotective effect of aminoguanidine in experimental diabetes cannot be reproduced by L-NAME or methylguanidine. It is likely that the effect of aminoguanidine is mediated predominantly by decreased AGE formation rather than via NOS inhibition. It also raises the possibility that inhibition of fluorescent AGE formation may be more renoprotective than inhibition of the formation of carboxymethyllysine-containing AGEs.

Effect of methylguanidine on rat blood pressure: role of endothelial nitric oxide synthase.

1. The effect of acute i.v. administration of methylguanidine (MG) on mean arterial blood pressure (MABP) was investigated in anaesthetized male Wistar rats. 2. MG (1-30 mg kg-1 i.v.) produced an increase in MABP in a dose-dependent manner both in normal and in hexamethonium (5 mg kg-1, i.v)-treated rats. 3. L-Arginine (30 or 150 mg kg-1, i.v.), but not its enantiomer D-arginine (30 or 150 mg kg-1, i.v.), reversed the effect of MG on MABP in both normal and hexamethonium-treated rats. 4. L-Arginine (150 mg kg-1, i.v.) administered 2 min before MG (30 mg kg-1, i.v.) prevented the increase in MABP caused by MG in either normal or hexamethonium-treated rats. This effect was not observed with D-arginine (150 mg kg-1, i.v.). 5. Thus, the rise in MABP caused by MG in the anaesthetized rat is due to inhibition of endothelial NO-synthase activity. We speculate that the rise in the plasma concentration of endogenous MG associated with uraemia may contribute to the hypertension seen in patients with chronic renal failure.

Meobentine sulfate: antiarrhythmic and electrophysiologic effects assessed by programmed electrical stimulation and ambulatory monitoring in patients with complex ventricular tachyarrhythmia. Report of a multicenter evaluation.

Five cardiology centers conducted open-label prospective trials of meobentine sulfate, an intravenously and orally available analog of bethanidine, to assess its potential for treatment of recurrent, drug refractory ventricular tachycardia (VT) or fibrillation (VF), and complex ventricular arrhythmias. The study population comprised 26 patients (mean age, 61 years); 18 were men. Coronary artery disease was present in 15, cardiomyopathy in six, and valvular heart disease in three. Patients presented with both VT and VF (seven), sustained VT alone (12), or frequent ventricular ectopy (PVCs) and nonsustained VT (seven). Of the 26 patients, 5 were enrolled in antiarrhythmic studies (chronic PVC suppression) and 21 were enrolled in programmed electrical stimulation (PES) studies. Two of five in the chronic PVC study showed greater than 75% arrhythmia suppression. Among 21 patients in PES studies, there were eight intravenous (16 mg/kg) and 19 oral trials (400 to 1000 mg every 6 hours, 3 days/dose interval). Five of 22 patients showed efficacy at repeat PES study (neither VT nor VF), one showed partial efficacy, and four were not restudied because of clinical arrhythmia (three) and/or adverse effects (two). Overall, three patients (12%) were continued on the drug for an extended period of time. Adverse experience included hypotension in 50% and gastrointestinal effects (nausea, vomiting, or diarrhea) in 56% (oral trials only). Adverse reactions led to drug discontinuation in six and dosage reduction in eight patients. Thus, meobentine may prevent induction of VT or VF or reduce frequency of complex PVCs in selected patients refractory to other antiarrhythmic agents, but the response rate is relatively low. Symptomatic hypotension or gastrointestinal adverse effects are common and may limit utility of meobentine as a chronic oral antiarrhythmic agent.