Potent antihyperglycaemic property of a new imidazoline derivative S-22068 (PMS 847) in a rat model of NIDDM

Bioanalytical method development for momordicinin and its application to long-term pharmacokinetics in diabetic rats

OBJECTIVE

To develop and validate bioanalytical RP-HPLC method to evaluate pharmacokinetics and tissue distribution pattern of momordicinin (MRN).

SIGNIFICANCE

MRN is one of the major cucurbitane triterpenoid found in Momordica charantia Linn (MC). However, MRN has not been explored for its pharmacokinetic profile, tissue distribution, and stability in order to establish it as an antidiabetic agent.

METHODS

In 28 days pharmacokinetic study, 54 diabetic male wistar rats were divided into three different groups and administered with 25, 50, and 100 mg/kg MRN orally. The blood samples were collected at 1, 7, 14, 21, and 28th day of the treatment and plasma quantification of MRN was done by validated RP-HPLC method. The rats were sacrificed at end of the study for tissue distribution.

RESULTS

The developed method was successfully applied to investigate pharmacokinetic profile of MRN. In pharmacokinetic analysis, the C_max for 25, 50, and 100 mg/kg was found to be 8.412, 10.443, and 11.829 µg/mL respectively suggesting the dose dependent activity. The maximum plasma concentration was achieved at 2 h for all doses. MRN showed major distribution in liver followed by kidney, spleen, and pancreas.

CONCLUSION

The newly developed and validated method was used to assay MRN in plasma as well as in tissues to evaluate pharmacokinetics of the drug for the first time. Undoubtedly, these findings can be taken into consideration while concluding its therapeutic effects after oral administration.

Rodent animal models: from mild to advanced stages of diabetic nephropathy

Diabetic nephropathy (DN) is a secondary complication of both type 1 and type 2 diabetes, resulting from uncontrolled high blood sugar. 30-40% of diabetic patients develop DN associated with a poor life expectancy and end-stage renal disease, causing serious socioeconomic problems. Although an exact pathogenesis of DN is still unknown, several factors such as hyperglycemia, hyperlipidemia, hypertension and proteinuria may contribute to the progression of renal damage in diabetic nephropathy. DN is confirmed by measuring blood urea nitrogen, serum creatinine, creatinine clearance and proteinuria. Clinical studies show that intensive control of hyperglycemia and blood pressure could successfully reduce proteinuria, which is the main sign of glomerular lesions in DN, and improve the renal prognosis in patients with DN. Diabetic rodent models have traditionally been used for doing research on pathogenesis and developing novel therapeutic strategies, but have limitations for translational research. Diabetes in animal models such as rodents are induced either spontaneously or by using chemical, surgical, genetic, or other techniques and depicts many clinical features or related phenotypes of the disease. This review discusses the merits and demerits of the models, which are used for many reasons in the research of diabetes and diabetic complications.

Design and synthesis of novel imidazoline derivatives with potent antihyperglycemic activity in a rat model of type 2 diabetes

Imidazoline derivatives have been reported to show antihyperglycemic activity in vivo. In the present study, we first showed that there was no correlation between the in vivo antidiabetic activity and the in vitro affinities for the I1/I2 binding sites for several substituted aryl imidazolines. Among these compounds, 2-(alpha-cyclohexyl-benzyl)-4,5-dihydro-1H-imidazole 2 exhibited potent antihyperglycemic properties. It was then chosen as lead compound. Thirty-six new derivatives were synthesized by replacing the cyclohexyl/benzyl group by various cyclic systems or the imidazoline ring by isosteric heterocycles. These compounds were evaluated in vivo for their antihyperglycemic activity using an oral glucose tolerance test (OGTT) in a rat model of type-2 diabetes obtained by giving a single intravenous (iv) injection of a low dose of streptozotocin to rats (STZ rats) and in normal rats. Nine compounds with an imidazoline moiety, possibly substituted by a methyl group, had a potent effect on the glucose tolerance in normal or STZ-diabetic rats, after an oral (po) administration of the test compound at a dose of 30 or 10 mg kg(-1), without any hypoglycemia. Replacement of the imidazoline ring by isosteric heterocycles resulted in a total loss of activity.

Imidazoline receptors: new targets for antihyperglycaemic drugs

In both animal models of Type 2 diabetes and in man, it has been evident for many years that certain imidazoline drugs can stimulate insulin secretion and improve glycaemia. This suggests that they may be useful new reagents for use in the management of Type 2 diabetes. However, despite their promise, no imidazoline compound has yet come into clinical use as an effective therapeutic agent in diabetes. This should not be taken as evidence of a flaw in the basic hypothesis, but derives, in part, from continuing ignorance about the molecular characteristics of imidazoline binding proteins, and the precise structure-activity relationships of their ligands. In this review, the mode of action of antihyperglycaemic imidazoline compounds is considered, and the possibility discussed that these agents may interact with a unique subtype of imidazoline binding site associated with ATP-sensitive potassium channels. The functional consequences of this interaction are summarised together with evidence that the binding site may actually lie within the channel complex. Additional data implicating the participation of alpha2-adrenoceptors in some actions of imidazolines are evaluated, and examples of relevant drugs having encouraging therapeutic profiles are highlighted. The possibility that some anti-diabetic imidazoline reagents may exert extra-pancreatic effects is also considered. Overall, the article aims to highlight important developments within the field but also draws attention to those areas where controversy remains.

Effects of the imidazoline ligands efaroxan and KU14R on blood glucose homeostasis in the mouse

The putative imidazoline I(3) receptor antagonist 2-(2-ethyl-2,3-dihydrobenzo[b]furan-2-yl)-1H-imidazole (KU14R) has been shown to block the effects of the atypical I(3) agonist efaroxan at the level of the ATP-sensitive K(+) (K(ATP)) channel in isolated pancreatic islet beta cells, but its effects in vivo are not known. We have therefore investigated the effects of KU14R on blood glucose and insulin level in vivo. When KU14R was administered before or after a hypoglycaemic dose of efaroxan, the fall in blood glucose was at least additive. When the antihyperglycaemic imidazoline ligand S22068 was administered after a dose of KU14R, it did not alter the hypoglycaemic response. In the mouse isolated vas deferens preparation, neither rauwolscine (at concentrations which competitively antagonised the inhibitory response to 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK14304)) nor KU14R affected inhibition produced by S22068. At 10(-4) M, KU14R had weak alpha(2)-adrenoceptor antagonist activity. We conclude that KU14R does not act as an antagonist of either efaroxan or S22068 at an imidazoline site in vivo.

Evidence that the novel imidazoline compound FT005 is an alpha(2)-adrenoceptor agonist

1: The aim of this study was to determine whether the hyperglycaemic action of the novel imidazoline compound FT005 could be mediated by activation of alpha(2)-adrenoceptors, using a variety of in vivo and in vitro methods including radioligand binding. 2: FT005 produced a dose-dependent increase in blood glucose levels of CBA/Ca mice (0.125-25 mg kg(-1), i.p.). The time course of this hyperglycaemic effect matched that of adrenaline (1 mg kg(-1)) more closely than glucagon (1 mg kg(-1)) or the K(ATP) channel opener diazoxide (25 mg kg(-1)). The hyperglycaemic effect of FT005 (1 mg kg(-1)) was significantly reduced by the alpha(2)-adrenoceptor antagonist rauwolscine (0.5 mg kg(-1)). 3: FT005 produced a significant reduction in plasma insulin levels of mice 30 min after administration. The hyperglycaemic effect of FT005 (25 mg kg(-1)), although still present, was significantly less in fasted mice in which insulin levels are lower, suggesting that a reduction of insulin secretion contributes to the action of FT005. 4: When studied in the mouse isolated vas deferens preparation, FT005 produced a complete inhibition of neurogenic contractions, which was blocked by rauwolscine. This is consistent with activation of pre-synaptic alpha(2)-adrenoceptors. 5: In radioligand binding studies FT005 completely displaced the alpha(2)-adrenoceptor antagonist [(3)H]-RX821002 from mouse whole brain homogenates. The displacement was best described by a two-site model of interaction comprising high and low affinity components. 6: The results indicate that FT005 is an agonist at alpha(2)-adrenoceptors. A reduction in insulin secretion contributes to the hyperglycaemic action of FT005, although an additional mechanism can not be excluded.

Specific desensitization of sulfonylurea- but not imidazoline-induced insulin release after prolonged tolbutamide exposure

Functional effects of prolonged exposure to the sulfonylurea, tolbutamide, were examined in the clonal electrofusion-derived BRIN-BD11 cell line. In acute 20-min incubations, 50-400 microM tolbutamide stimulated a dose-dependent increase (P < 0.01) in insulin release at both non-stimulatory (1.1 mM) and stimulatory (8.4 mM) glucose. Culture with 100 microM tolbutamide (18 hr) caused a marked (67%) decrease in subsequent insulin-secretory responsiveness to acute challenge with 200 microM tolbutamide, though notably, tolbutamide culture exerted no influence on 200 microM efaroxan-induced insulin secretion. Duration of exposure (3-18 hr) to 100 microM tolbutamide in culture also time-dependently influenced subsequent responsiveness to acute tolbutamide challenge, with progressive 47-58% decreases from 6-18 hr (P < 0.001). Similarly, 6- to 18-hr culture with 100 microM efaroxan specifically desensitized efaroxan-induced insulin release. Tolbutamide- and efaroxan-induced desensitization exhibited a time-dependent reversibility, with a sustained return to full insulin-secretory responsiveness by 12 hr. Notably, 18-hr culture with tolbutamide or efaroxan did not significantly affect insulinotropic responses to 16.7 mM glucose, 10 mM 2-ketoisocaproic acid, 10 mM alanine, 10 mM arginine, or 30 mM KCl. Diverse inhibitory actions of tolbutamide or efaroxan culture on late events in stimulus-secretion coupling reveal that drug desensitization is both a specific and important phenomenon. As such, the model system described could prove an important tool in determining the complex modes of action of established and novel clinically useful insulinotropic compounds.

Glucose-induced sympathetic activity and energy expenditure during acute alpha2-adrenergic antagonism in obese subjects

OBJECTIVE

To determine the effect of an alpha2-adrenoceptor antagonist, idazoxan, on the sympathetic nervous system and on energy expenditure responses after an oral glucose load, in obese patients. (idazoxan acts as an indirect sympathomimetic drug through blockade of presynaptic alpha2-adrenoceptors).

DESIGN

Double-blind randomized placebo-controlled cross-over study. Idazoxan (40 mg) or placebo were administered orally 90 min before a 100 g oral glucose load.

SUBJECTS

Twelve long-standing obese subjects (six men and six women, age range from 24 to 45 y, body mass index range from 30.2 to 41.3 kg/m2).

MEASUREMENTS

Energy expenditure was derived from oxygen consumption and carbon dioxide production according to indirect calorimetry. Plasma samples were obtained for plasma adrenaline and noradrenaline, glucose, non-esterified fatty acid (NEFA), glycerol and insulin determinations.

RESULTS

The plasma noradrenaline concentration response to the glucose load was significantly higher after idazoxan than after placebo administration. The time-course of glucose load-induced thermogenesis was not significantly different after administration of idazoxan nor placebo. Idazoxan administration did not modify the insulin, non-esterified fatty acids or glycerol concentration responses to the glucose load. Neither heart rate nor blood pressure values were modified by idazoxan when compared to placebo. However, idazoxan significantly improved glucose tolerance.

CONCLUSION

The alpha2-adrenergic antagonist idazoxan increases glucose-induced sympathetic activity but not energy expenditure in obese subjects. These data do not argue for the development of alpha2AR antagonist compounds as anti-obesity treatment.

Effect of acute and sub-chronic administration of the imidazoline compound S 22068 on in vivo glucose and insulin responses in normal lean CBA/Ca mice

Acute S 22068 (24 mg/kg po) improved glucose tolerance and increased insulin sensitivity, assessed as the acute blood glucose response to exogenous insulin. The same acute dose did not stimulate insulin secretion or induce hypoglycemia in fed animals. Comparison of acute S 22068 to equipotent doses (with respect to effect on glucose tolerance) of gliclazide (2 mg/kg) and metformin (60 mg/kg) found S 22068 to be similar to metformin with respect to its effects on basal glucose levels (BGL) and insulin sensitivity. This also suggests that S 22068 acts by a mechanism which does not involve insulin release. Acute or sub-chronic S 22068 (14 days at 25 mg/day) had no effect on brown adipose tissue (BAT) or white adipose tissue (WAT) lipogenesis, an insulin-sensitive metabolic pathway. Sub-chronic treatment with S 22068 did not alter body weight (BW) or food intake, and resulted in tolerance to its effects on glucose metabolism and insulin sensitivity. These findings suggest that S 22068 is similar in effect to metformin, and is not insulinogenic, in contrast to the sulfonylureas or putative I(3) imidazoline site ligands.

In vitro mechanism of action on insulin release of S-22068, a new putative antidiabetic compound

1. The MIN6 cell line derived from in vivo immortalized insulin-secreting pancreatic beta cells was used to study the insulin-releasing capacity and the cellular mode of action of S-22068, a newly synthesized imidazoline compound known for its antidiabetic effect in vivo. 2. S-22068, was able to release insulin from MIN6 cells in a dose-dependent manner with a half-maximal stimulation at 100 micronM. Its efficacy (8 fold over the basal value), which did not differ whatever the glucose concentration (stimulatory or not), was intermediate between that of sulphonylurea and that of efaroxan. 3. Similarly to sulphonylureas and classical imidazolines, S-22068 blocked K(ATP) channels and, in turn, opened nifedipine-sensitive voltage-dependent Ca2+ channels, triggering Ca2+ entry. 4. Similarly to other imidazolines, S-22068 induced a closure of cloned K(ATP) channels injected to Xenopus oocytes by interacting with the pore-forming Kir6.2 moiety. 5. S-22068 did not interact with the sulphonylurea binding site nor with the non-I1 and non-I2 imidazoline site evidenced in the beta cells that is recognized by the imidazoline compounds efaroxan, phentolamine and RX821002. 6. We conclude that S-22068 is a novel imidazoline compound which stimulates insulin release via interaction with an original site present on the Kir6.2 moiety of the beta cell K(ATP) channels.

Effect of the new imidazoline derivative S-22068 (PMS 847) on insulin secretion in vitro and glucose turnover in vivo in rats

We have investigated the possible mechanisms underlying the antihyperglycaemic effect of the imidazoline derivative S-22068. In vitro, in the presence of 5 mmol/l glucose, S-22068 (100 micromol/l) induced a significant and sustained increase in insulin secretion from isolated, perifused, rat islets and a marked sensitization to a subsequent glucose challenge (10 mmol/l). S-22068 (100 micromol/l was able to antagonize the stimulatory effect of diazoxide on 86Rb efflux from preloaded islets incubated in the presence of 20 mmol/l glucose. Experiments were also performed to investigate whether S-22068 can alter glucose turnover and peripheral insulin sensitivity in vivo in mildly diabetic rats and obese, insulin resistant, Zucker rats. Neither glucose production nor individual tissue glucose utilization was modified by S-22068 in either group of rats. Similar results were obtained whether the studies were performed under basal conditions or during euglycaemic/hyperinsulinemic clamps. The results suggest that S-22068 exerts part of its antihyperglycaemic effect by promoting insulin secretion without alteration of peripheral insulin sensitivity.

Multiple effector pathways regulate the insulin secretory response to the imidazoline RX871024 in isolated rat pancreatic islets

When isolated rat islets were cultured for 18 h prior to use, the putative imidazoline binding site ligand, RX871024 caused a dose-dependent increase in insulin secretion at both 6 mM and 20 mM glucose. By contrast, a second ligand, efaroxan, was ineffective at 20 mM glucose whereas it did stimulate insulin secretion in response to 6 mM glucose. Exposure of islets to RX871024 (50 microM) for 18 h, resulted in loss of responsiveness to this reagent upon subsequent re-exposure. However, islets that were unresponsive to RX871024 still responded normally to efaroxan. The imidazoline antagonist, KU14R, blocked the insulin secretory response to efaroxan, but failed to prevent the stimulatory response to RX871024. By contrast with its effects in cultured islets, RX871024 inhibited glucose-induced insulin release from freshly isolated islets. Efaroxan did not inhibit insulin secretion under any conditions studied. In freshly isolated islets, the effects of RX871024 on insulin secretion could be converted from inhibitory to stimulatory, by starvation of the animals. Inhibition of insulin secretion by RX871024 in freshly isolated islets was prevented by the cyclo-oxygenase inhibitors indomethacin or flurbiprofen. Consistent with this, RX871024 caused a marked increase in islet PGE2 formation. Efaroxan did not alter islet PGE2 levels. The results suggest that RX871024 exerts multiple effects in the pancreatic beta-cell and that its effects on insulin secretion cannot be ascribed only to interaction with a putative imidazoline binding site.

Imidazolines and pancreatic hormone secretion

A range of imidazoline derivatives are known to be effective stimulators of insulin secretion, and this response correlates with closure of ATP-sensitive potassium channels in the pancreatic beta-cell. However, mounting evidence indicates that potassium channel blockade may form only part of the mechanism by which imidazolines exert their effects on insulin secretion. Additionally, it remains unclear whether members of this class of drugs can bind directly to potassium channel components and whether occupation of a single binding site accounts for their functional activity. This review considers recent developments in the field and highlights evidence that does not fit readily with the concept that a single mechanism of action is sufficient to mediate the effects of imidazolines on pancreatic hormone secretion.

Design and synthesis of imidazoline derivatives active on glucose homeostasis in a rat model of type II diabetes. 2. Syntheses and biological activities of 1,4-dialkyl-, 1,4-dibenzyl, and 1-benzyl-4-alkyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazines and isosteric analogues of imidazoline

Piperazine derivatives have been identified as new antidiabetic compounds. Structure-activity relationship studies in a series of 1-benzyl-4-alkyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazines resulted in the identification of 1-methyl-4-(2', 4'-dichlorobenzyl)-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazine, PMS 812 (S-21663), as a highly potent antidiabetic agent on a rat model of diabetes, mediated by an important increase of insulin secretion independently of alpha2 adrenoceptor blockage. These studies were extended to find additional compounds in these series with improved properties. In such a way, substitution of both piperazine N atoms was first optimized by using various alkyl, branched or not, and benzyl groups. Second, some modifications of the imidazoline ring and its replacement by isosteric heterocycles were carried out, proceeding from PMS 812, to evaluate their influence on the antidiabetic activity. The importance of the distance between the imidazoline ring and the piperazine skeleton was studied third. Finally, the influence of the N-benzyl moiety was also analyzed compared to a direct N-phenyl substitution. The pharmacological evaluation was performed in vivo using glucose tolerance tests on a rat model of type II diabetes. The most active compounds were 1,4-diisopropyl-2-(4', 5'-dihydro-1'H-imidazol-2'-yl)piperazine (41a), PMS 847 (S-22068), and 1,4-diisobutyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazine (41b), PMS 889 (S-22575), which strongly improved glucose tolerance without any side event or hypoglycemic effect. More particularly, PMS 847 proved to be as potent after po (100 micromol/kg) as after ip administration and appears as a good candidate for clinical investigations.