The Journey of Allopurinol

Allopurinol is well absorbed orally with approximately 80-90%
bioavailability. Peak plasma concentrations of allopurinol are reached within 1-1.5 hours, while
peak concentrations of its primary active metabolite, oxypurinol, occur at 3-5 hours. Food slightly
delays absorption without significantly reducing overall bioavailability; the drug may be taken with
or without food. The drug is available in 100 mg and 300 mg tablets. Patients with gout are typically
started at 100 mg/day (to avoid precipitating a gout flare from rapid urate mobilization) and
titrated upward. Despite modest allopurinol plasma levels (parent half-life only 1-2 hours), the
prolonged oxypurinol plasma concentrations (half-life 14-28 hours) provide sustained xanthine
oxidase inhibition enabling once-daily dosing. Intravenous allopurinol sodium is available for
tumor lysis syndrome prevention when oral administration is not possible.

Allopurinol distributes widely into body fluids with a volume
of distribution of approximately 1.6 L/kg. Plasma protein binding of allopurinol is negligible
(<3%), while oxypurinol is modestly bound to plasma proteins (~10-12%). Both compounds distribute
readily into cells, with intracellular concentrations matching plasma levels. Oxypurinol accumulates
in tissues with high xanthine oxidase activity — primarily the liver and small intestinal mucosa
where xanthine oxidase is most abundantly expressed. CSF penetration of allopurinol is limited
but some CNS distribution occurs, explaining rare neurological adverse effects. Allopurinol and
oxypurinol distribute into breast milk; safety in breastfeeding is considered acceptable given low
concentrations.

Uric acid is the end product of purine catabolism in humans (humans
lack uricase). Xanthine oxidase (XO, also xanthine dehydrogenase, encoded by XDH) catalyzes the
last two steps: hypoxanthine → xanthine → uric acid, each step producing hydrogen peroxide and
superoxide. Allopurinol is a structural isomer of hypoxanthine. It is first hydroxylated by xanthine
oxidase to oxypurinol (alloxanthine), which is an isomer of xanthine. Oxypurinol then binds the
active site of xanthine oxidase in a reduced form, forming a stable covalent complex with molybdenum
at the catalytic center — inhibiting the enzyme with essentially irreversible (tight-binding) kinetics.
This suicide substrate mechanism means that the enzyme is only regenerated by protein turnover.
Blocking uric acid production diverts purine catabolism to the more soluble metabolites xanthine
and hypoxanthine (also inhibited by salvage pathway enzymes), reducing serum urate from a typical
gout range of >8 mg/dL to targets of <6 mg/dL (standard) or <5 mg/dL (tophaceous gout).

The primary metabolism is XO-mediated conversion of allopurinol
to oxypurinol — the same enzyme allopurinol is designed to inhibit. This creates a pharmacokinetic
feedback loop: as XO is inhibited by increasing oxypurinol concentrations, the conversion of
remaining allopurinol to oxypurinol slows. Minimal CYP450 metabolism occurs. Oxypurinol is also
an XO substrate, though it is oxidized much more slowly than allopurinol. Important drug interaction:
allopurinol dramatically increases plasma concentrations of azathioprine and mercaptopurine (both
inactivated by XO); co-administration requires dose reduction of azathioprine/6-MP to 25-33% of
the usual dose or complete avoidance. Allopurinol also inhibits the metabolism of theophylline
(by XO) and warfarin (by CYP2C9 inhibition). Probenecid increases oxypurinol renal clearance,
reducing efficacy.

Allopurinol is rapidly cleared from plasma (half-life 1-2 hours)
primarily by conversion to oxypurinol. Oxypurinol is excreted unchanged by the kidney via glomerular
filtration and tubular reabsorption; its half-life in patients with normal renal function is 14-28 hours.
Renal impairment dramatically reduces oxypurinol clearance, leading to accumulation and increased
risk of toxicity — the most severe being allopurinol hypersensitivity syndrome (AHS), a rare but
life-threatening reaction involving severe skin reactions (SJS/TEN), hepatitis, eosinophilia,
and renal failure. Dose reduction proportional to creatinine clearance is mandatory in CKD.
Oxypurinol is dialyzable; hemodialysis patients require dosing after dialysis sessions. HLA-B*5801
allele (predominantly in Han Chinese, Korean, and Thai populations) is a major pharmacogenomic
risk factor for AHS, prompting guideline recommendations for genetic screening.

Allopurinol is the most widely used urate-lowering therapy globally
for gout, hyperuricemia, uric acid nephrolithiasis, and tumor lysis syndrome prevention. Initiating
allopurinol can paradoxically trigger gout flares (mobilization of urate from deposits) — co-
prophylaxis with colchicine or NSAIDs for at least 3-6 months is recommended. Febuxostat is an
alternative non-purine XO inhibitor for patients intolerant of allopurinol or with HLA-B*5801
positivity. Target serum urate levels: <6 mg/dL (standard), <5 mg/dL for tophaceous gout. Allopurinol
also has a cytoprotective role (reduces reactive oxygen species from XO) that is under investigation
in cardiovascular disease and ischemia-reperfusion injury.