Baseline Renal Urate Reabsorption via URAT1
Most filtered urate returns to blood through the URAT1 transporter.
- ~1100 mg/day: Filtered urate load (GFR × plasma urate)
- ~90%: URAT1 reabsorption (apical anion exchanger)
- 3.5–7.2 mg/dL: Normal serum urate (physiologic range)
- ~600 mg/day: Baseline excretion (net renal output)
URAT1 and the proximal tubule urate transport cycle
URAT1 exchanges luminal urate for intracellular anions, pulling urate back into blood.
Filtration, secretion, and net reabsorption balance
Urate is freely filtered, largely reabsorbed, then partly resecreted downstream.
Net renal handling keeps most people's serum urate below the crystallization threshold.
Probenecid Administered — URAT1 Blockade
Probenecid competitively inhibits URAT1-mediated urate reabsorption.
- URAT1 inhibitor: Probenecid MOA (competitive blockade)
- 500–2000 mg/day: Typical dose (divided doses)
- up to 65%: Reabsorption blocked (dose-dependent)
- OAT1 / OAT3: Also inhibits (organic anion transporters)
Competitive inhibition of the URAT1 anion exchanger
Probenecid occupies the transporter's binding site, preventing urate uptake.
Dose-response relationship and transporter occupancy
Higher doses block a larger fraction of URAT1 transport sites.
Higher doses of probenecid progressively unmask more filtered urate for excretion.
Increased Urinary Urate Excretion
Blocked reabsorption diverts more filtered urate straight into urine.
- 2–3×: Excretion increase (vs untreated baseline)
- ~30 min: Onset of effect (peak effect 2–4 h)
- rises sharply: Fractional excretion (FEurate %)
- increases: Renal urate clearance (CLurate)
Fractional excretion of urate as a clinical marker
FEurate rises as fewer reabsorption sites remain functional.
Time course of uricosuric drug action
Excretion climbs quickly after dosing, then plateaus near steady state.
Serum Urate Declines
Sustained uricosuria gradually lowers circulating serum urate.
- 1–2 mg/dL: Serum urate drop (typical reduction)
- ~1 week: Time to steady state (chronic dosing)
- gout, hyperuricemia: Clinical use (uricosuric therapy)
- renal impairment: Key contraindication (GFR below ~30)
Mass balance between excretion and serum concentration
Sustained excretion above production slowly depletes the urate pool.
Clinical use in gout and chronic hyperuricemia
Lower serum urate reduces future crystal deposition in joints.
Probenecid is generally avoided when kidney function is already reduced.
Nephrolithiasis Risk vs Hydration / Urine Alkalinization
Concentrated, acidic urine turns excreted urate into a stone risk.
- low urine pH: Main stone risk driver (uric acid is insoluble)
- hydration + alkalinization: Mitigation strategy (citrate, bicarbonate)
- >2 L/day: Target urine output (dilutes urate load)
- 6.0–6.5: Target urine pH (raises urate solubility)
Uric acid solubility and the pKa-driven crystallization threshold
Below its pKa, uric acid is poorly soluble and prone to crystallize.
Hydration and urinary alkalinization as protective strategies
More urine volume and higher pH both cut crystallization risk.
Adequate fluids and mild alkalinization let uricosuric therapy proceed safely.