Minoxidil

A K+ channel-opening vasodilator originally approved for severe hypertension that grew hair as a side effect, then got repurposed twice — first as topical Rogaine (1988, OTC since 1996), now off-label as low-dose oral minoxidil (LDOM, ~2017+) for MPB. Mechanism on hair is partial mystery — anagen prolongation + follicular vascularization, gated by individual scalp SULT1A1 sulfation activity (~30-40% of users are non-responders). For Dylan at 20 with no hair loss, this is treatment for a problem he doesn't have. SKIP-FOR-NOW; canonical first-line if MPB appears.

Minoxidil is a prodrug. The molecule itself is largely inactive — it must be sulfated by sulfotransferase SULT1A1 to minoxidil sulfate, which is the active species. This sulfation happens both hepatically (for systemic effects) and locally in the outer root sheath of hair follicles (for hair effects). Inter-individual variation in scalp SULT1A1 activity is the leading explanation for the responder/non-responder split in topical use.

Cardiovascular mechanism (Loniten / oral):

• Minoxidil sulfate opens ATP-sensitive potassium channels (K_ATP) in vascular smooth muscle.
• K+ efflux → membrane hyperpolarization → reduced Ca²⁺ influx → smooth muscle relaxation → arteriolar (not venous) vasodilation.
• Drops peripheral vascular resistance hard. At hypertensive doses (5-40 mg/day) this triggers reflex tachycardia, sodium/water retention, and pericardial effusion in a small fraction — which is why oral minoxidil for hypertension is reserved for severe refractory cases requiring concurrent beta-blocker + diuretic.
• At LDOM doses (0.25-5 mg/day) the cardiovascular signal is much smaller — most users tolerate without beta-blocker/diuretic, but blood pressure, heart rate, and ankle edema warrant baseline + periodic check.

Hair mechanism (incomplete picture):

• Anagen prolongation: minoxidil sulfate extends the growth phase of the hair cycle, so individual hairs spend more time growing and reach greater terminal length/diameter before shedding.
• Follicle enlargement: miniaturized follicles in androgenetic alopecia partially re-thicken under chronic minoxidil; the size/diameter of the hair shaft increases.
• Vascularization: dermal papilla blood flow increases — this was the original intuition (the drug is a vasodilator, more blood = more growth) but is now thought to be supportive rather than causal.
• Wnt/β-catenin signaling: minoxidil sulfate activates this pathway in dermal papilla cells in vitro, promoting anagen induction. Plausible mechanism but not fully validated in vivo.
• Prostaglandin effects: some evidence minoxidil modulates prostaglandin synthesis at the follicle, though less central than for latanoprost-class compounds.
• What it does NOT do: minoxidil does not affect DHT levels or androgen receptor signaling at all. It is mechanistically orthogonal to finasteride/dutasteride (5α-reductase inhibitors) and ru58841 (AR antagonist) — which is exactly why combining them is standard practice in MPB treatment.

SULT1A1 polymorphism — why some users don't respond:

• Scalp SULT1A1 enzyme activity is highly variable between individuals (genetic + acquired factors).
• Low-activity individuals generate insufficient minoxidil sulfate locally → minimal hair response despite full topical dosing.
• Commercial assays exist (Hairgenix, Daniel Alain "Minoxidil Response Test" via SULT1A1 activity) but real-world predictive value is mediocre — many "non-responders" by assay still show some clinical response, and vice versa.
• Topical tretinoin co-application has been shown to upregulate SULT1A1 activity in scalp in some users, partially rescuing non-responders. Not universally effective.