A type 2 diabetic patient on metformin and a GLP-1 receptor agonist achieves inadequate glycaemic control. A DPP-4 inhibitor is considered next. Why is adding a DPP-4 inhibitor to a GLP-1 RA not recommended?
- A DPP-4 inhibitors work by preventing GLP-1 degradation, so their mechanism is redundant when a GLP-1 RA is already providing supraphysiological GLP-1 receptor stimulation ✓
- B Both drugs cause significant weight gain when combined, worsening metabolic syndrome
- C The combination causes severe hypoglycaemia by synergistically increasing insulin secretion
- D DPP-4 inhibitors are contraindicated in patients on renin-angiotensin system agents
Correct answer: A. DPP-4 inhibitors work by preventing GLP-1 degradation, so their mechanism is redundant when a GLP-1 RA is already providing supraphysiological GLP-1 receptor stimulation
Explanation
DPP-4 inhibitors act by inhibiting the enzyme that degrades endogenous GLP-1 and GIP, raising native incretin levels modestly (achieving physiological GLP-1 concentrations). GLP-1 receptor agonists directly stimulate GLP-1 receptors at supraphysiological levels, bypassing endogenous GLP-1. Adding a DPP-4 inhibitor confers no additional benefit in this situation, making the combination redundant.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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