Oral hypoglycaemic agent according to harrison textbook

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Oral Hypoglycaemic Agents - Harrison's Principles of Internal Medicine, 22nd Edition (2025)

Overview

Pharmacologic treatment of type 2 DM involves multiple classes of agents. The initial step is to select an individualized glycemic goal. In most non-pregnant adults, the ADA target is HbA1c <7%, with a more stringent target of ≤6.5% for selected patients, and a higher target of <7.5-8% for elderly, cognitively impaired, or hypoglycemia-unaware patients.

1. Biguanides - Metformin

Mechanism:

Primarily reduces hepatic glucose production (gluconeogenesis and glycogenolysis) by activating AMP-activated protein kinase (AMPK) via inhibition of mitochondrial complex I
Also reduces insulin resistance in peripheral tissues (muscle, fat)
Slightly delays intestinal glucose absorption

Pharmacokinetics:

Half-life ~2 hours; excreted unchanged in urine
Available as standard and extended-release (XR) formulations

Dosing:

Start at 500 mg once or twice daily with meals; increase to 850-1000 mg twice daily as tolerated
Maximum effective dose: ~2000 mg/day (doses above this add little benefit but more GI side effects)

HbA1c reduction: ~1-2%

Advantages:

No weight gain or hypoglycemia as monotherapy
Reduces cardiovascular events (UKPDS data)
Inexpensive
Proven long-term safety record

Side effects:

GI: nausea, diarrhea, cramping (most common, dose-related)
Lactic acidosis (rare, primarily in patients with renal failure, hepatic disease, hypoxia, or excess alcohol use)
Vitamin B12 deficiency (long-term use, monitor annually)

Contraindications:

eGFR <30 mL/min/1.73 m² (hold for contrast procedures if eGFR <60)
Hepatic failure, active alcohol abuse, conditions predisposing to lactic acidosis

Position: First-line agent for type 2 DM in most patients (unless contraindicated)

2. Sulfonylureas (SUs)

Mechanism: Bind to ATP-sensitive K⁺ channels (SUR1 subunit) on pancreatic beta cells → close the channel → membrane depolarization → calcium influx → insulin secretion. Effect is glucose-independent (hence risk of hypoglycemia).

Examples and dosing:

Drug	Dose Range	Duration	Notes
Glipizide	2.5-40 mg/day	Short-acting	Preferred in elderly; less hypoglycemia
Glyburide (glibenclamide)	1.25-20 mg/day	Long-acting	Higher hypoglycemia risk; avoid in elderly/renal disease
Glimepiride	1-8 mg/day	Once daily	Less hypoglycemia than glyburide
Gliclazide	80-320 mg/day	Intermediate	Preferred in some guidelines

HbA1c reduction: ~1-2%

Advantages:

Potent glucose lowering
Inexpensive
Decades of clinical experience

Side effects:

Hypoglycemia (most important - can be prolonged, especially glyburide)
Weight gain (1-3 kg)
Rare: cholestatic jaundice, agranulocytosis, rash, hyponatremia (especially with chlorpropamide)

Contraindications: Renal/hepatic failure (increased hypoglycemia risk); type 1 DM; ketoacidosis

3. Meglitinides (Non-sulfonylurea Secretagogues)

Mechanism: Similar to SUs - close K⁺-ATP channels on beta cells, but bind at a different site. Stimulate rapid, short-duration insulin release.

Examples:

Repaglinide (0.5-4 mg with each meal, up to 3x/day)
Nateglinide (60-120 mg with each meal)

HbA1c reduction: ~1-1.5%

Advantages:

Rapid onset, short duration → taken with meals → flexibility (skip if skipping meal)
Better for controlling postprandial spikes
Repaglinide can be used with some degree of renal impairment

Side effects:

Hypoglycemia (less than SUs due to short action)
Weight gain

4. Thiazolidinediones (TZDs) - Glitazones

Mechanism: Bind peroxisome proliferator-activated receptor gamma (PPARγ) - a nuclear receptor - altering transcription of genes involved in glucose and lipid metabolism → increases insulin sensitivity in muscle, fat, and liver (reduces insulin resistance). Do NOT stimulate insulin secretion.

Examples:

Pioglitazone (15-45 mg once daily)
Rosiglitazone (use restricted due to cardiovascular concerns)

HbA1c reduction: ~1-1.5%

Advantages:

Durable glucose lowering
Pioglitazone reduces cardiovascular events in high-risk patients (PROactive trial)
No hypoglycemia as monotherapy
Pioglitazone improves NAFLD/NASH

Side effects:

Weight gain (fat redistribution - subcutaneous increase, visceral decrease)
Fluid retention / edema → worsens or precipitates heart failure
Bone fractures (increased risk in women - peripheral fractures)
Bladder cancer risk (pioglitazone, dose- and duration-dependent)
Slow onset of action (weeks)
Macular edema (rare)

Contraindications: Heart failure (NYHA class III-IV), hepatic disease, bladder cancer, pregnancy

5. Alpha-Glucosidase Inhibitors

Mechanism: Inhibit intestinal brush-border alpha-glucosidases (enzymes that digest complex carbohydrates) → delay carbohydrate absorption → reduce postprandial glucose excursions. No systemic absorption; work locally in the gut.

Examples:

Acarbose (25-100 mg with each meal)
Miglitol
Voglibose

HbA1c reduction: ~0.5-0.8% (modest)

Advantages:

No systemic side effects
No hypoglycemia as monotherapy
Reduces postprandial hyperglycemia
Modest weight reduction

Side effects:

GI: flatulence, diarrhea, abdominal bloating (very common - due to undigested carbohydrates reaching the colon; limit dose titration)
Elevated liver enzymes (rare, with high-dose acarbose)

Contraindications: Inflammatory bowel disease, malabsorption syndromes, renal failure (creatinine >2 mg/dL)

Note: If hypoglycemia occurs in a patient on acarbose combination therapy, treat with glucose (dextrose), not sucrose - because sucrose absorption will be delayed.

6. DPP-4 Inhibitors (Gliptins)

Mechanism: Inhibit dipeptidyl peptidase-4 (DPP-4), the enzyme that degrades incretins (GLP-1 and GIP) → prolongs action of endogenous GLP-1/GIP → glucose-dependent insulin secretion + reduced glucagon. Because action is glucose-dependent, hypoglycemia risk is very low.

Examples:

Drug	Dose	Renal Dose Adjustment
Sitagliptin	100 mg once daily	Yes (reduce with low eGFR)
Saxagliptin	2.5-5 mg once daily	Yes
Alogliptin	25 mg once daily	Yes
Linagliptin	5 mg once daily	No (hepatically excreted)
Vildagliptin	50 mg twice daily	Yes

HbA1c reduction: ~0.5-1%

Advantages:

Weight neutral
Low hypoglycemia risk
Well tolerated
Cardiovascular safety established (TECOS, SAVOR, EXAMINE trials)

Side effects:

Nasopharyngitis, upper respiratory infections
Pancreatitis (rare but reported)
Saxagliptin: modest increase in heart failure hospitalization (caution in heart failure)
Possible increased risk of bullous pemphigoid

7. SGLT-2 Inhibitors (Gliflozins)

Mechanism: Inhibit sodium-glucose cotransporter 2 (SGLT-2) in the proximal renal tubule → block ~90% of renal glucose reabsorption → glycosuria → blood glucose lowering, osmotic diuresis, and caloric loss. Insulin-independent mechanism.

Examples:

Drug	Dose
Empagliflozin	10-25 mg once daily
Canagliflozin	100-300 mg once daily
Dapagliflozin	5-10 mg once daily
Ertugliflozin	5-15 mg once daily

HbA1c reduction: ~0.5-1%

Key cardiovascular and renal benefits (beyond glucose lowering):

Empagliflozin (EMPA-REG OUTCOME): Reduced CV death, hospitalization for heart failure, progression of renal disease
Canagliflozin (CREDENCE): Reduced renal endpoints in diabetic CKD
Dapagliflozin (DAPA-HF): Reduces HF hospitalizations and CV death even in non-diabetics

Advantages:

Weight loss (2-3 kg)
Blood pressure reduction (3-5 mmHg systolic)
Cardioprotective and nephroprotective - preferred in T2DM + ASCVD + CKD or heart failure
No hypoglycemia as monotherapy

Side effects:

Genital mycotic infections (most common - due to glycosuria)
UTIs
Euglycemic diabetic ketoacidosis (especially perioperative, sick day; may occur with normal blood glucose)
Fournier's gangrene (necrotizing fasciitis of the perineum - rare but serious)
Volume depletion / hypotension (in elderly or on diuretics)
Canagliflozin: Lower limb amputations (increased risk), bone fractures

Contraindications: eGFR <30-45 (depending on agent and indication); type 1 DM (risk of euglycemic DKA); hold perioperatively

8. GLP-1 Receptor Agonists

(While primarily injectable, oral semaglutide is available and classified as an oral hypoglycemic agent)

Mechanism: Activate GLP-1 receptors → glucose-dependent insulin secretion, suppress glucagon, slow gastric emptying, reduce appetite/food intake.

Oral agent:

Oral Semaglutide (Rybelsus): 3 mg → 7 mg → 14 mg once daily, taken fasting 30 min before eating

HbA1c reduction: ~1-1.5%

Advantages:

Significant weight loss
Cardiovascular benefit (SUSTAIN-6, PIONEER-6 trials for semaglutide)
No hypoglycemia as monotherapy
Preferred in T2DM + ASCVD

Side effects: Nausea, vomiting, diarrhea (dose-dependent, improve over time), pancreatitis (rare), contraindicated in personal/family history of medullary thyroid carcinoma or MEN2

9. Bile Acid Sequestrants

Colesevelam (3.75 g/day) - modest HbA1c reduction (~0.4-0.5%)
Mechanism of glucose lowering not fully understood
Also lowers LDL cholesterol
Side effects: constipation, hypertriglyceridemia (contraindicated if TG >500 mg/dL)

10. Dopamine Agonist

Bromocriptine-QR (quick-release) - 0.8-4.8 mg once daily (morning)
Modest HbA1c reduction (~0.5%)
Mechanism: thought to reset circadian rhythm and hypothalamic dopaminergic tone
Side effects: nausea, dizziness, fatigue

Summary Table

Class	Example	HbA1c ↓	Hypoglycemia	Weight	Key Benefit	Key Risk
Biguanide	Metformin	1-2%	No	Neutral/↓	CV benefit, cheap	GI side effects, lactic acidosis
Sulfonylurea	Glipizide	1-2%	Yes	↑	Potent, cheap	Hypoglycemia
Meglitinide	Repaglinide	1-1.5%	Yes (less)	↑	Postprandial control	Multiple daily doses
TZD	Pioglitazone	1-1.5%	No	↑	Durable, NASH benefit	CHF, fractures
Alpha-glucosidase inhibitor	Acarbose	0.5-0.8%	No	Neutral/↓	Postprandial glucose	Flatulence
DPP-4 inhibitor	Sitagliptin	0.5-1%	No	Neutral	Well tolerated	Pancreatitis (rare)
SGLT-2 inhibitor	Empagliflozin	0.5-1%	No	↓	CV/renal protection	Genital infections, DKA
GLP-1 RA (oral)	Semaglutide	1-1.5%	No	↓↓	CV benefit, weight loss	GI side effects

Treatment Algorithm (Harrison's / ADA Framework)

Start metformin (unless contraindicated) + lifestyle modification
If HbA1c not at goal after 3 months, add a second agent based on comorbidities:
- ASCVD or high CV risk: Add GLP-1 RA or SGLT-2 inhibitor
- Heart failure or CKD: Prefer SGLT-2 inhibitor
- Weight loss needed: GLP-1 RA or SGLT-2 inhibitor
- Hypoglycemia risk: Avoid sulfonylureas; prefer DPP-4i, GLP-1 RA, SGLT-2i, TZD
- Cost concern: Sulfonylurea or TZD
Progress to triple therapy or insulin if dual therapy fails

Source: Harrison's Principles of Internal Medicine, 22nd Edition (2025), Chapter 416 - Pharmacologic Treatment of Diabetes

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