Synthesis and in vitro antidiabetic activity of some alkyl carbazole compounds

Purpose: To synthesize some alkyl derivatives of carbazole and evaluate their in vitro inhibitory effect on alpha amylase and alpha glucosidase. Methods: Synthesis of methylcarbazole, ethylcarbazole, propylcarbazole and butylcarbazole was carried out using acid-catalysed alkylation method while in vitro inhibitory assay on alpha amylase and alpha glucosidase enzymes on the synthesized compounds was evaluated using standard procedures. Acarbose was used as the reference compound. Results: For carbazole, methylcarbazole, ethylcarbazole, propylcarbazole and butylcarbazole, the IC50 values of alpha amylase inhibitory assay were 87.47, 50.23, 47.20, 42.36 and 42.11 μg/mL respectively. IC50 values of alpha glucosidase inhibitory assay for ethylcarbazole, propylcarbazole and butylcarbazole were 205.30, 153.93 and 152.90 μg/mL, respectively. Carbazole and methylcarbazole had no inhibitory effect on this enzyme but the reference drug (acarbose) had a better inhibitory effect towards the two enzymes than the synthesized products. Conclusion: Some alkyl-carbazoles with anti-diabetic effect have been successfully synthesised. Alkylation of carbazole increased the alpha amylase inhibitory effect of carbazole. The inhibitory effect is directly proportional to the chain length of the alkyl group.


INTRODUCTION
Diabetes mellitus (DM) is a metabolic disorder [1] which is the fourth main cause of death in most developed countries and one of the leading causes of kidney failure, heart attack, blindness and lower limb amputation.It has been projected by International Diabetes Federation that about 330 million people will be diabetic by the year 2025, if adequate measures are not taken, and over 75 % of this will be in developing countries as compared with 62 % in 1995 [2].One promising therapeutic approach is to decrease hyperglycaemia by decreasing the postprandial rise in blood glucose concentration.This is achieved by retarding and reducing the digestion and absorption of ingested carbohydrates through the inhibition of carbohydrate hydrolyzing enzymes such as α-amylase and α-glucosidase [3].Thus, the search for new leads for antidiabetic drugs with lower cost and better efficiency has therefore become a matter of major priority.
Carbazole and its derivatives are important type of nitrogen containing heterocyclic compounds that are widespread in nature [4].The chemistry and biology of carbazole have attracted an increasing interest over the last 50 years.The presence of a desirable electronic and charge transport properties, and also large pieconjugated system in carbazole make the introduction of various functional groups into the structurally rigid carbazolyl ring easy, resulting in the extensive potential application of carbazole in the field of medicinal chemistry [5].Some carbazole derivatives are known to possess anticancer, antimicrobial, anti HIV, antihypertensive, antiparkinsonian, antipsychotic, anti-emetic and antidiabetic activities [6].Some antidiabetic compounds such as tetrahydroalstonine, lochnerinine, vindoline, harmine, reserpine, vincanidine and vinervine obtained from plants have carbazole-like nucleus which may be essential to their antidiabetic activity.
This work was therefore undertaken to synthesise and test some alkyl-carbazole compounds for their antidiabetic activity by evaluating their inhibitory effects on alpha amylase and alpha glucosidase activities.This could provide a 'lead' to the discovery of novel antidiabetic drugs.

Synthesis of alkylcarbazole compounds
Alkylation of carbazole was carried out using acid catalysed synthesis method: carbazole (2.5g, 10 mmol) was dissolved in conc.Sulphuric acid (40 mL) and stirred vigorously in a round-bottomed flask for 2 h using a magnetic stirer followed by addition of iodoalkane (0.1 mol).The reaction medium was suspended in ice and stirring continued for 5 h.This was rinsed into 100 mL iced water using 10 mL conc.sulphuric acid and stirred for 30 min in a cool water bath.100 mL of conc.ammonia was further added to basify the medium and was allowed to stand for 2 h.The cooled reaction medium was extracted using 50 mL acetone and was volatilized at room temperature to obtain crude alkylcarbazole.
Each of the synthesized compounds (10 mg) was dissolved in 25 mL of cyclohexane and pH of the solution was determined using Luton Electronic pH meter.The melting pint was also determined using the Electrothermal Melting Point Apparatus (England.CAT.No. 1A6304).
The synthesized compounds were each dissolved in chloroform and the absorbance was measured at wavelength 200-400 nm using Spectro UV-VIS Dual Beam (8 Auto Cell UVS-270; Lambomed, Inc, USA).FTIR spectra of the synthesized compounds were obtained using FTIR-8400S Fourier Transform Infrared Spectrophotometer.
Ten milligram each of synthesized compound was analysed in GCMS equipment (GCMS-QP2010 PLUS) for molecular weight elucidation with the following acquisition parameters: Oven temperature programme (temperature and hold time, 70

Evaluation of alpha-amylase inhibitory activity
Assay was carried out according to the method reported by Sindhu et al [7].The assay mixture containing 200 µL of 0.02 M sodium phosphate buffer, 20 µL of enzyme and the synthesized products in concentration range 20-100 µg/mL were incubated for 10 minutes at room temperature followed by addition of 200 µL of starch in all test tubes.The reaction was terminated with addition of 400 µL dinitrosalicyclic acid reagent and placed in boiling water bath for 5 min, cooled and diluted with 15 mL of distilled water and absorbance was measured at 540 nm.The control sample was prepared without any synthesized product.The percentage Inhibition was calculated as: The concentration of alkylcarbazole required to inhibit 50 % of α-amylase activity was defined by the IC 50 value determined from plot of percent inhibition versus log inhibitor concentration and was calculated by nonlinear regression analysis from the mean inhibitory values.Acarbose was used as the reference alpha-amylase inhibitor (positive control) and all tests were performed in triplicate.

Evaluation of alpha-glucosidase inhibitory activity
Alpha glucosidase inhibition assay was performed as reported by Adefegha and Oboh [8].Alpha glucosidase was dissolved in 100 mM phosphate buffer (pH =6.8) and was used as the enzyme extract.p-Nitrophenyl-α-Dglucopyranoside was used as the substrate.Synthesized product were used in the concentration ranging from 20-100 µg/mL.Different concentrations of synthesized products were mixed with 320 µL of 100 mM phosphate buffer for 5 min.3 mL of 50 mM sodium hydroxide was added to the mixture and the absorbance was read at 410 nm.The control samples were prepared without any synthesized products.The percentage inhibition was calculated as: The concentration of alkylcarbazole require to inhibit 50% of α-amylase activity was defined by the IC 50 value determined from plot of percent inhibition versus log inhibitor concentration and was calculated by nonlinear regression analysis from the mean inhibitory values.Acarbose was used as the reference alpha-amylase inhibitor (positive control) and all tests were performed in triplicate.

DISCUSSION
Methylcarbazole, ethylcarbazole, propylcarbazole and butylcarbazole have been successfully synthesized by alkylation of carbazole.In vitro inhibitory assay of synthesized products on alpha amylase showed a significant increase in inhibitory effect of the alkylcarbazole derivatives with increasing chain length of alkyl group.
The IC 50 value of carbazole, methylcarbazole, ethylcarbazole, propylcarbazole, butylcarbazole and acarbose (reference drug) decreased with increase in chain length of alkyl groups.This shows that inhibitory effect towards alpha amylase and hence antidiabetic activity increased with increase in alkyl group chain length (Figure 1).The alpha amylase inhibitory activity of propyl and butyl carbazole was comparable to that of Acarbose.For alphaglucosidase inhibitory activity, the IC 50 values are 205.30,153.93, 152.90 and 59.57 µg/mL for ethylcarbazole, propylcarbazole, butylcarbazole and acarbose (reference drug) respectively.Carbazole, methylcarbazole and 20 µg/mL ethylcarbazole did not inhibit the activity of alpha glucosidase (Figure 2).These results showed that the inhibitory effect of ethylcarbazole (40 -100 µg/mL), propylcarbazole and butylcarbazole as compared with the reference drug (Acarbose) was not significant.
One way of reducing hyperglycaemia is to inhibit the activities of alpha amylase and alpha glucosidase.This is because pancreatic alpha amylase catalyses the breaking down of starch into disaccharides and oligosaccharides while intestinal alpha glucosidase catalyzes the breakdown of disaccharides to release glucose which is later absorbed from small intestine into the blood circulation [9].The alkyl carbazole compounds synthesized in this work inhibited alpha amylase activity and therefore may have the potential of reducing hyperglycaemia.

CONCLUSION
Four alkyl-carbozoles (methyl-, ethyl-, propyland butyl-carbazoles) with anti-diabetic effect have been successfully synthesised.These compounds have significant inhibitory activities against alpha amylase but alpha glucosidase.Alkylation of carbazole increases the alpha amylase inhibitory effect of carbazole.This inhibitory effect is directly proportional to the chain length of the alkyl group.Alkyl carbazole compounds could therefore serve as lead for novel alpha amylase inhibitory and hence antidiabetic compounds.