Synthesis of Biguanides and Guanamines and Their Effect on Glycogen Phosphorylase a and Steroid Metabolism in Cultured Hepatocytes From Normal and Streptozotocin Induced Diabetic Rats

Since the introduction of beta-phenethylbiguanide (DBI)) as an outstanding blood sugar lowering drug, considerable attention has been directed towards the synthesis of compounds which are in some way related to biguanides as potential anti-diabetic compounds. It may be noted that the biguanides when written in cyclic hydrogen bonded form (I), resemble a triazine-like structure (II). On the basis of the consideration as to whether stabilisation of the biguanide group by ring formation could enhance the anti diabetic activity of these compounds, a series of substituted s-triazine derivatives was synthesised and tested as oral hypoglycaemics in various animal models. [diagram] Guanamine-like compounds have previously been shown to lower blood glucose levels in normal as well as diabetic rats. In the present study a series of biguanide and guanamine derivatives were synthesised and tested for their ability to decrease glycogen phosphorylase a in hepatocytes isolated from non- diabetic and streptozotocin-induced (STZ) diabetic rats, as a measure of their insulin-like action. The biguanides such as the phenyl-, benzyl- and phenethyl (phenformin, a therapeutically useful) derivatives were cyclized into guanamines to see the effect of cyclization on the activity of the compound. It was also possible to determine the effect of changing substituents in various positions and to determine the structure-activity relationship of these compounds synthesised for their insulin-like action. All of the results can be summarised as follows.(1). In hepatocytes isolated from control animals, (a) all of the synthesised biguanides and guanamines mimic insulin in decreasing glycogen phosphorylase. (b) in the biguanide series, the benzyl and phenyl derivatives were more potent than phenethylbiguanide (phenformin). (c) cyclization of the biguanide to the corresponding 1,2,3-triazines (guanamines) still gives compounds which inhibit the activity of glycogen phosphorylase. (2). In hepatocytes isolated from diabetic rats, all compounds were seen to increase glycogen phosphorylase a activity in contrast to the inhibition seen in cells from control animals. The most potent compounds were phenethylbiguanide (phenformin) and phenethylguanamine. This effect is opposite to that seen in hepatocytes from control animals and may be compensation for the inhibition of glycogen phosphorylase activity seen in liver from diabetic animals which is, perhaps, due to the low amount of glycogen seen in the liver of these animals. In contrast to the situation in control hepatocytes, diabetic hepatocytes showed the greatest response to phenethylbiguanide and only a moderate response to benzylbiguanide. As in the control animals, however, biguanides and guanamines mimic insulin and thus, the guanamines may prove useful as antidiabetics. (3). The effect of biguanides and guanamines on glycogen phosphorylase a in the insulin treated diabetic rat was much more like the effect of the compounds on normal rat hepatocytes. (4). The next series of experiments was performed to try and elucidate the reason for the differences in effects of compounds on glycogen phosphorylase activity in different types of cells by preincubation of normal rat hepatocytes in different concentrations of glucose. A similar structure-activity relationship was observed in control cells preincubated in 30mM glucose as in cells isolated from STZ diabetic rats. (5) It was observed that all of the compounds mimic insulin in stimulating the activity of glycogen phosphorylase when the diabetic cells are preincubated in 10mM glucose and by decreasing glycogen phosphorylase activity in cells preincubated in 30 mM glucose. This result suggests that cells isolated from STZ diabetic rats are not normal and respond differently to cells isolated from control liver. (6). The insulin-mimetic effects of phenethylbiguanide and phenethylguanamine in increasing androst-4-ene-3,17-dione metabolism were examined in order to extend our knowledge of the insulin-like action of biguanides and guanamines from glycogen metabolism to steroid metabolism. Phenethylbiguanide and phenethylguanamine are both able to mimic the effect of insulin in normal rat hepatocytes with respect to androst-4-ene-3,17-dione metabolism. However, the cyclization of phenethylbiguanide to phenethylguanamine gives a compound which is only effective on the cytochrome P450- dependent enzymes (7alpha- 6beta- and 16alpha-hydroxylases). This possibly indicates that the three cytochrome P450 dependent enzymes are more susceptible than the flavin-dependent enzymes to the triazine moiety in phenethylguanamine. Overall, biguanides and guanamines have been shown to have significant, direct, insulin-like effects on glycogen phosphorylase a activity and androst-4-ene-3,17-dione metabolism in hepatocytes isolated from control and STZ-diabetic rats. The effectiveness of the compounds depended on the enzyme under study and on the original state of the animal (diabetic or not). Further investigation is warranted on the usefulness of congeners of the presently used biguanide, phenformin, and on the cyclised biguanides, such as the guanamines and, perhaps, the five-membered structures, the triazoles