Oral antidiabetic agents: current role in type 2 diabetes mellitus.

摘要

Type 2 diabetes mellitus is a progressive and complex disorder that is difficult to treat effectively in the long term. The majority of patients are overweight or obese at diagnosis and will be unable to achieve or sustain near normoglycaemia without oral antidiabetic agents; a sizeable proportion of patients will eventually require insulin therapy to maintain long-term glycaemic control, either as monotherapy or in conjunction with oral antidiabetic therapy. The frequent need for escalating therapy is held to reflect progressive loss of islet beta-cell function, usually in the presence of obesity-related insulin resistance. Today's clinicians are presented with an extensive range of oral antidiabetic drugs for type 2 diabetes. The main classes are heterogeneous in their modes of action, safety profiles and tolerability. These main classes include agents that stimulate insulin secretion (sulphonylureas and rapid-acting secretagogues), reduce hepatic glucose production (biguanides), delay digestion and absorption of intestinal carbohydrate (alpha-glucosidase inhibitors) or improve insulin action (thiazolidinediones). The UKPDS (United Kingdom Prospective Diabetes Study) demonstrated the benefits of intensified glycaemic control on microvascular complications in newly diagnosed patients with type 2 diabetes. However, the picture was less clearcut with regard to macrovascular disease, with neither sulphonylureas nor insulin significantly reducing cardiovascular events. The impact of oral antidiabetic agents on atherosclerosis--beyond expected effects on glycaemic control--is an increasingly important consideration. In the UKPDS, overweight and obese patients randomised to initial monotherapy with metformin experienced significant reductions in myocardial infarction and diabetes-related deaths. Metformin does not promote weight gain and has beneficial effects on several cardiovascular risk factors. Accordingly, metformin is widely regarded as the drug of choice for most patients with type 2 diabetes. Concern about cardiovascular safety of sulphonylureas has largely dissipated with generally reassuring results from clinical trials, including the UKPDS. Encouragingly, the recent Steno-2 Study showed that intensive target-driven, multifactorial approach to management, based around a sulphonylurea, reduced the risk of both micro- and macrovascular complications in high-risk patients. Theoretical advantages of selectively targeting postprandial hyperglycaemia require confirmation in clinical trials of drugs with preferential effects on this facet of hyperglycaemia are currently in progress. The insulin-sensitising thiazolidinedione class of antidiabetic agents has potentially advantageous effects on multiple components of the metabolic syndrome; the results of clinical trials with cardiovascular endpoints are awaited. The selection of initial monotherapy is based on a clinical and biochemical assessment of the patient, safety considerations being paramount. In some circumstances, for example pregnancy or severe hepatic or renal impairment, insulin may be the treatment of choice when nonpharmacological measures prove inadequate. Insulin is also required for metabolic decompensation, that is, incipient or actual diabetic ketoacidosis, or non-ketotic hyperosmolar hyperglycaemia. Certain comorbidities, for example presentation with myocardial infarction during other acute intercurrent illness, may make insulin the best option. Oral antidiabetic agents should be initiated at a low dose and titrated up according to glycaemic response, as judged by measurement of glycosylated haemoglobin (HbA1c) concentration, supplemented in some patients by self monitoring of capillary blood glucose. The average glucose-lowering effect of the major classes of oral antidiabetic agents is broadly similar (averaging a 1-2% reduction in HbA1c), alpha-glucosidase inhibitors being rather less effective. Tailoring the treatment to the individual patient is an impo