Oral Hypoglycemic Drugs (Antidiabetic Oral Agents)

Overview of Drug Classes

1. Biguanides - Metformin

• Mechanism: Acts primarily by reducing hepatic gluconeogenesis, thereby decreasing hepatic glucose output. It activates AMP-activated protein kinase (AMPK). It works even in the absence of insulin, unlike sulfonylureas.
• Additional uses: Polycystic ovary syndrome (PCOS), prevention of T2DM in prediabetes
• Pharmacokinetics: Oral, renally excreted unchanged (no hepatic metabolism)
• Adverse effects:
  • GI disturbances (nausea, diarrhea, metallic taste) - most common; reduced by taking with food or using extended-release form
  • Lactic acidosis - rare but serious; risk increased in renal impairment, hepatic disease, alcohol use, or contrast administration
  • Vitamin B12 deficiency with prolonged use
• Contraindications: eGFR < 30 mL/min, hepatic impairment, alcoholism, use of iodinated contrast (hold perioperatively)
• Does NOT cause hypoglycemia or weight gain - major advantage

2. Sulfonylureas

• Mechanism: Bind to the ATP-sensitive K⁺ channel (SUR1 subunit) on pancreatic β cells, blocking K⁺ efflux. This depolarizes the β cell membrane, opens voltage-gated Ca²⁺ channels, increases intracellular Ca²⁺, and triggers insulin release - independent of plasma glucose level.
• Requirement: Need functioning β cells; ineffective after pancreatectomy or in type 1 diabetes
• Adverse effects:
  • Hypoglycemia - most important (especially with missed meals, alcohol, renal impairment)
  • Weight gain
  • Hyponatremia (particularly chlorpropamide - due to SIADH-like effect)
  • Disulfiram-like reaction with alcohol (chlorpropamide)
• 2nd gen preferred over 1st gen due to greater potency, fewer drug interactions, shorter half-lives

3. Meglitinides (Non-sulfonylurea Secretagogues)

• Mechanism: Similar to sulfonylureas - bind to K⁺-ATP channels but at a different site, stimulate insulin release
• Advantage: Shorter duration of action - taken with meals to control postprandial glucose; repaglinide can be used in renal impairment
• Adverse effects: Hypoglycemia (less than sulfonylureas), weight gain

4. Thiazolidinediones (TZDs / Glitazones)

• Mechanism: Agonists at Peroxisome Proliferator-Activated Receptor-γ (PPARγ), a nuclear transcription factor. Activation increases transcription of insulin-responsive genes, increasing insulin sensitivity in adipose tissue, liver, and skeletal muscle. They do not stimulate insulin secretion.
• Onset: Slow - maximum effect may take 6-12 weeks
• Adverse effects:
  • Fluid retention → edema and worsening of heart failure (avoid in NYHA Class III/IV)
  • Weight gain (increased subcutaneous fat)
  • Osteopenia and fracture risk (especially in women)
  • Pioglitazone: increased risk of bladder cancer
  • Rosiglitazone: boxed warning for increased risk of myocardial infarction (less commonly used now)
  • Liver toxicity (monitor LFTs)
• Does NOT cause hypoglycemia as monotherapy

5. DPP-4 Inhibitors (Gliptins)

• Mechanism: Inhibit the enzyme dipeptidyl peptidase-4 (DPP-4), which normally degrades incretin hormones (GLP-1 and GIP). Prolonging incretin activity increases:
  • Glucose-stimulated insulin release
  • Reduction of inappropriate glucagon secretion
• Key feature: Effect is glucose-dependent (only active when glucose is elevated) - very low hypoglycemia risk
• Adverse effects:
  • Generally well tolerated
  • Nasopharyngitis, upper respiratory infections
  • Pancreatitis (rare but reported)
  • Saxagliptin: possible increased risk of hospitalization for heart failure
• Weight neutral
• Do not combine with GLP-1 receptor agonists (overlapping mechanism, no added benefit)

6. SGLT2 Inhibitors (Gliflozins)

• Mechanism: Inhibit sodium-glucose cotransporter 2 (SGLT2) in the proximal renal tubule, which normally reabsorbs ~90% of filtered glucose. Inhibition causes glucosuria (urinary glucose excretion), lowering blood glucose independently of insulin.
• Additional benefits (beyond glycemia):
  • Cardiovascular: Reduce mortality and hospitalizations for heart failure in patients with AND without T2DM (empagliflozin approved for both HFrEF and HFpEF; dapagliflozin for HFrEF)
  • Renal: Slow progression of chronic kidney disease
  • Weight loss (caloric loss via glycosuria)
  • Blood pressure reduction (natriuresis)
• Mechanism in heart failure: Inhibit cardiac sodium-hydrogen exchanger (NHE), reduce glucose utilization for ATP, reduce preload/afterload
• Adverse effects:
  • Genital mycotic infections (most common - due to glycosuria)
  • Urinary tract infections
  • Euglycemic diabetic ketoacidosis (rare but serious - can occur even with normal glucose)
  • Fournier's gangrene (rare necrotizing fasciitis of perineum)
  • Canagliflozin: increased risk of lower limb amputation, bone fractures
  • Polyuria, volume depletion
• Contraindicated in eGFR < 30 (no efficacy); require adequate renal function

7. α-Glucosidase Inhibitors

• Mechanism: Competitively inhibit intestinal α-glucosidase enzymes (maltase, sucrase, glucoamylase), delaying digestion and absorption of complex carbohydrates → reduces postprandial glucose rise
• Must be taken with the first bite of each meal
• Adverse effects: GI - flatulence, diarrhea, abdominal cramping (from colonic fermentation of unabsorbed carbohydrates) - very common and limits use
• Low hypoglycemia risk as monotherapy
• If hypoglycemia occurs while on acarbose (in combination therapy), treat with glucose (dextrose) NOT sucrose (sucrose absorption is blocked)

8. Other/Miscellaneous Agents

Bromocriptine (Cycloset)

• Dopamine D2 agonist; reduces insulin resistance; modest HbA1c reduction; approved for T2DM

Colesevelam

• Bile acid sequestrant; modest glucose-lowering effect via unclear mechanism

GLP-1 Receptor Agonists (injectable, with oral semaglutide)

• Technically injectable for most, but oral semaglutide is an oral option
• Mechanism: Mimic GLP-1 - stimulate insulin secretion (glucose-dependent), suppress glucagon, delay gastric emptying, promote satiety
• Agents: Exenatide, Liraglutide, Dulaglutide, Semaglutide (oral and SC), Lixisenatide
• Major benefits: Weight loss, CV risk reduction (MACE reduction with liraglutide, semaglutide), approved in HFrEF
• Adverse effects: Nausea, vomiting, diarrhea (especially early); pancreatitis; thyroid C-cell tumors (contraindicated in medullary thyroid cancer/MEN2)

Treatment Algorithm (Type 2 DM)

1. First-line: Metformin (unless contraindicated) + lifestyle modification
2. Dual therapy: Add agent based on patient profile:
   • Heart failure or CKD → add SGLT2 inhibitor (or GLP-1 RA)
   • Atherosclerotic CVD → add GLP-1 RA or SGLT2 inhibitor
   • Need to minimize hypoglycemia → DPP-4 inhibitor, TZD, or SGLT2 inhibitor
   • Need weight loss → GLP-1 RA or SGLT2 inhibitor
   • Cost concern → Sulfonylurea
3. Triple therapy: Multiple combinations as needed
4. Insulin: Added when oral agents insufficient; sulfonylureas/meglitinides/TZDs may be discontinued when starting insulin

Key Comparisons

• Lippincott Illustrated Reviews: Pharmacology, Chapter 24 (Diabetes drugs)
• Ganong's Review of Medical Physiology, 26th Edition
• Katzung's Basic and Clinical Pharmacology, 16th Edition