Synthalin, Buformin, Phenformin, and Metformin: A Century of Intestinal “Glucose Excretion” as Oral Antidiabetic Strategy in Overweight/Obese Patients

After the first release of synthalin B (dodecamethylenbiguanide) in 1928 and its later retraction in the 1940s in Germany, the retraction of phenformin (N-Phenethylbiguanide) and of Buformin in the USA (but not outside) because of the lethal complication of acidosis seemed to have put an end to the era of the biguanides as oral antidiabetics. The strongly hygroscopic metformin (1-1-dimethylbiguanide), first synthesized 1922 and resuscitated as an oral antidiabetic (type 2 of the elderly) compound first released in 1959 in France and in other European countries, was used in the first large multicenter prospective long-term trial in England in the UKPDS (1977–1997). It was then released in the USA after a short-term prospective trial in healthy overweight “young” type 2 diabetics (mean age 53 years) in 1995 for oral treatment of type 2 diabetes. It was, however, prescribed to mostly multimorbid older patients (above 60–65 years of age). Metformin is now the most used oral drug for type 2 diabetes worldwide. While intravenous administration of biguanides does not have any glucose-lowering effect, their oral administration leads to enormous increase in their intestinal concentration (up to 300-fold compared to that measured in the blood), to reduced absorption of glucose from the diet, to increased excretion of glucose through the stool, and to decrease in insulin serum level through increased hepatic uptake and decreased production. Intravenously injected F18-labeled glucose in metformin-treated type 2 diabetics accumulates in the small and even more in the large intestine. The densitometry picture observed in metformin-treated overweight diabetics is like that observed in patients after bowel-cleansing or chronically taking different types of laxatives, where the accumulated radioactivity can even reach values observed in colon cancer. The glucose-lowering mechanism of action of metformin is therefore not only due to inhibition of glucose uptake in the small intestine but also to “attraction” of glucose from the hepatocyte into the intestine, possibly through the insulin-mediated uptake in the hepatocyte and its secretion into the bile. Furthermore, these compounds have also a diuretic effect (loss of sodium and water in the urine) Acute gastrointestinal side effects accompanied by fluid loss often lead to the drugs’ dose reduction and strongly limit adherence to therapy. Main long-term consequences are “chronic” dehydration, deficiency of vitamin B12 and of iron, and, as observed for all the biguanides, to “chronic” increase in fasting and postprandial lactate plasma level as a laboratory marker of a clinical condition characterized by hypotension, oliguria, adynamia, and evident lactic acidosis. Metformin is not different from the other biguanides: synthalin B, buformin, and phenformin. The mechanism of action of the biguanides as antihyperglycemic substances and their side effects are comparable if not even stronger (abdominal pain, nausea, vomiting, diarrhea, fluid loss) to those of laxatives.