Drug Metabolism, Distribution and Metabolite Identification

The hepatic endoplasmic reticulum carries a family of haem-containing mono-oxygenase enzymes called cytochrome P450, named for the 450 nm absorption peak of the reduced carbon-monoxide complex. Around 57 functional CYP genes exist, but only six isoenzymes carry the majority of forensic drug metabolism. CYP3A4 is the single most important, responsible for roughly half of marketed drugs including most benzodiazepines (alprazolam, midazolam), statins, opioids (fentanyl, methadone, oxycodone), calcium channel blockers and the antiretroviral protease inhibitors. A patient on ketoconazole, clarithromycin or ritonavir has CYP3A4 inhibited, which raises concentrations of any co-administered substrate to toxic levels. The drug-drug interaction profile shows up on autopsy when an elderly polypharmacy patient dies of an apparent overdose at a therapeutic dose.

CYP2D6 is the second pillar of forensic interest because it carries a wide genetic polymorphism. Around 7 percent of north Europeans are poor metabolisers (PM); around 1 to 5 percent of Indians are ultra-rapid metabolisers (UM) with gene duplications producing two to thirteen functional copies. CYP2D6 activates codeine to morphine and tramadol to O-desmethyltramadol. A UM child given codeine for post-tonsillectomy pain converts a large fraction to morphine and can die of respiratory depression. The FDA issued a black-box warning in 2013, and the Indian Academy of Pediatrics now advises against codeine in children under 12.

CYP2C9 handles warfarin, phenytoin and most NSAIDs. CYP1A2 handles caffeine, theophylline and clozapine, and is induced by cigarette smoke; a heavy smoker on clozapine who quits may develop toxicity within days as induction is lost. CYP2C19 activates clopidogrel and is inhibited by omeprazole. The reactions catalysed across the CYP family are aromatic and aliphatic hydroxylation, N-dealkylation (diazepam to nordiazepam), O-dealkylation (codeine to morphine), oxidative deamination (amphetamines), and sulphoxidation. Reduction (chloral hydrate to trichloroethanol) and hydrolysis (cocaine to benzoylecgonine, heroin to 6-MAM, aspirin to salicylic acid) also count as Phase I.

Phase II reactions attach a polar endogenous group to the drug or to its Phase I metabolite, converting a moderately lipophilic molecule into a water-soluble conjugate ready for renal or biliary excretion. The dominant pathway is glucuronidation by the UDP-glucuronosyltransferase (UGT) family. Morphine is conjugated at the 3-hydroxyl to morphine-3-glucuronide (inactive, dominant urinary metabolite) and at the 6-hydroxyl to morphine-6-glucuronide (a potent agonist responsible for much of the analgesic and respiratory depressant activity in chronic users). Paracetamol-glucuronide is the major paracetamol route.

Sulphation by the SULT family is a high-affinity low-capacity pathway. Paracetamol-sulphate is the second route, which saturates at high doses and shunts the excess parent to the CYP2E1-mediated NAPQI pathway. Glutathione conjugation, catalysed by the GST family, detoxifies reactive electrophiles including NAPQI from paracetamol and the epoxides of benzene and vinyl chloride. Glutathione depletion is what turns a paracetamol overdose into fulminant hepatic necrosis.

Acetylation, catalysed by N-acetyltransferase 2 (NAT2), is the polymorphic Phase II pathway with the strongest Indian forensic relevance. Cohort studies from AIIMS Delhi and PGIMER Chandigarh place slow-acetylator prevalence in north India at 55 to 65 percent, higher than south India or East Asia. Slow acetylators on isoniazid (the RNTCP anti-tubercular cornerstone) accumulate parent drug, develop peripheral neuropathy without pyridoxine co-supplementation, and run higher hepatotoxicity risk. Slow acetylators on hydralazine develop drug-induced lupus; on dapsone they accumulate parent drug and face higher methaemoglobinaemia risk. Other Phase II routes include methylation (catecholamines via COMT, thiopurines via TPMT) and glycine conjugation (salicylates to salicyluric acid).

A handful of metabolite identifications carry disproportionate weight in Indian forensic toxicology because they answer a medico-legal question that no parent concentration can answer. The single most cited is 6-monoacetylmorphine, the transient intermediate in the deacetylation of heroin to morphine. Heroin loses one acetyl group within minutes to give 6-MAM, the second over the next hour to give morphine. The 6-MAM blood half-life is six to twenty-five minutes, so a positive 6-MAM identification proves the source of the morphine was heroin and not pharmaceutical morphine or codeine. In a Punjab or Delhi heroin overdose, blood morphine and 6-MAM together secure the conviction under NDPS Section 21, where morphine alone would let the defence argue medical morphine.

Cocaine is hydrolysed by plasma and hepatic carboxylesterases to benzoylecgonine (loss of the methyl ester) and ecgonine methyl ester. Benzoylecgonine is the dominant urinary metabolite, half-life around six hours in blood and detectable in urine for two to three days after a single use, a week or longer in chronic users. Parent cocaine clears from urine in four to six hours. A urine drug screen that reports cocaine positive is in fact reporting BE, because the cocaine immunoassay runs against the metabolite, not the parent.

Ethanol metabolism follows two sequential steps: alcohol dehydrogenase (ADH) oxidises ethanol to acetaldehyde, and aldehyde dehydrogenase (ALDH) oxidises acetaldehyde to acetic acid. The disulfiram reaction blocks ALDH and lets acetaldehyde accumulate, producing the flushing and nausea behind alcohol aversion therapy. Methanol follows the same chain to formaldehyde and then formic acid. Formic acid, not methanol, is the toxicant driving the optic neuritis and metabolic acidosis of methanol poisoning. In methanol poisoning, fomepizole or ethanol competitively blocks ADH while haemodialysis clears parent methanol and accumulated formate, the approach used in major poisoning incidents including Mumbai Malad 2015 and Tarn Taran 2020.

Ethylene glycol, the antifreeze ingredient and a recurring suicidal poison in Indian truck-driver communities, is oxidised by ADH through glycolaldehyde, glycolic acid and glyoxylic acid to oxalic acid. Oxalate combines with calcium to form crystals that precipitate in renal tubules and produce acute oxalate nephropathy, with envelope-shaped crystals visible on urine microscopy. Calcium oxalate crystals in renal tubular lumens are the histopathological marker of ethylene glycol poisoning.

The codeine-to-morphine ratio requires explicit interpretation. A blood sample with codeine higher than morphine (ratio above 1) is consistent with codeine intake (cough syrup, codeine analgesic tablets), because parent codeine accumulates relative to the morphine product of CYP2D6 O-dealkylation. Morphine higher than codeine, or morphine with absent codeine, points to heroin or pharmaceutical morphine, and prompts a search for 6-MAM.

Vd is an apparent volume calculated as dose divided by the plasma concentration extrapolated back to time zero. It is not an anatomical volume. A drug with Vd of 0.1 L/kg (warfarin, salicylate) stays in plasma because of high protein binding. A drug with Vd of 5 to 10 L/kg (fentanyl, methadone, the tricyclics, digoxin) distributes into adipose, myocardial and lipid-rich depots, has a low plasma concentration relative to total body burden, and a long terminal half-life because slow release from depots rate-limits elimination.

The standard relationship is elimination half-life t½ equals 0.693 times Vd divided by clearance. Other routinely reported parameters are Cmax, Tmax, AUC and bioavailability F. Drugs with extensive first-pass metabolism (morphine, propranolol, verapamil) have low oral bioavailability, which is why the same dose produces a much higher concentration by the intravenous or sublingual route.

Post-mortem redistribution is the consequence of the high-Vd phenomenon after death. In life, tissue depots equilibrate with plasma through active transport and cardiac output. After death, cell membranes lose integrity and drug diffuses passively into the central blood compartments. For a basic lipophilic high-Vd drug stored in myocardium and pulmonary tissue, cardiac blood concentrations can rise two to ten times above the ante-mortem value within 24 to 48 hours. The classic PMR drugs are digoxin (five to tenfold rise), amitriptyline and nortriptyline (two to fivefold), fentanyl, methadone, propranolol and clozapine.

The forensic countermeasure is the matrix sampled. Peripheral femoral blood, drawn with a proximal clamp to prevent backflow of central blood, is the least affected by PMR and is the recommended matrix for quantitative interpretation. Cardiac blood should never be used alone for PMR-prone drugs. Vitreous humor, sequestered in the bony orbit, gives a relatively stable matrix for ethanol, sodium, urea, glucose and some drugs. Bile concentrates conjugated metabolites and confirms chronic morphine, methadone or cocaine use. Hair gives a temporal record at the 1 cm per month growth rate and is the matrix of choice for chronic exposure and older drug-facilitated assault investigations.

The parent-to-metabolite ratio is the basis for statements about route and timing of exposure. Immediately after an acute dose the parent is high and the metabolites are low, giving a high parent-to-metabolite ratio. As time passes the parent clears while metabolites accumulate (if their half-life is longer) or clear in parallel. At steady state the ratio reflects the relative half-lives and routes of formation.

For cocaine, parent far higher than benzoylecgonine indicates intake within the past one to two hours. BE far higher than parent, or BE alone with cocaine below detection, indicates intake more than four to six hours earlier or chronic use. Parent cocaine with no BE is suspicious for post-mortem contamination or in vitro spike. For heroin the timing runs through 6-MAM, half-life six to twenty-five minutes: a 6-MAM identification in blood proves heroin intake within the past hour. Morphine without 6-MAM in a suspected heroin death points to a longer survival interval. Urine 6-MAM extends the window to about 24 hours.

For methadone, EDDP identification proves methadone intake within recent days. A high methadone with low EDDP suggests a recent dose; a stable parent-metabolite ratio suggests steady-state opioid agonist therapy. For benzodiazepines, the diazepam to nordiazepam to oxazepam to temazepam cascade dates the dose. Recent diazepam intake shows parent high with metabolites trailing. High nordiazepam and oxazepam with absent parent suggests older or chronic dosing. Nordiazepam (half-life 50 to 100 hours) persists long after parent diazepam clears.

The interpretation also speaks to route. 6-MAM proves the heroin route (intravenous, smoked, insufflated) because medical morphine cannot produce it. BE proves cocaine regardless of route and separates genuine cocaine use from coca-leaf tea exposure. For ethanol, ethyl glucuronide and ethyl sulphate (minor Phase II conjugates) persist two to three days after the parent ethanol clears and are the standard biomarkers for alcohol use monitoring in workplace and clinical settings.

The published therapeutic, toxic and lethal ranges (Baselt, Disposition of Toxic Drugs and Chemicals in Man) are population guidelines from case series, not individual sentences. Three factors shift these boundaries. Tolerance in chronic opioid and benzodiazepine users raises the lethal threshold five to twentyfold; a chronic heroin user can survive a blood morphine of 1 microgram per millilitre that would kill an opioid-naive patient at 200 ng/mL. Idiosyncratic response from CYP and NAT2 polymorphisms produces toxicity at therapeutic doses (CYP2D6 UM on codeine, slow acetylator on isoniazid) or therapeutic failure at standard doses (CYP2C19 PM on clopidogrel). Polypharmacy and CYP3A4 inhibition by macrolides, azole antifungals and grapefruit juice can raise substrate concentrations two to fivefold.

Indian casework consistently illustrates the importance of metabolite identification. A 2017 heroin overdose autopsy at AIIMS Forensic Medicine showed blood morphine 480 ng/mL and 6-MAM 12 ng/mL. The defence argued the morphine could have been pharmaceutical, ingested deliberately to fabricate a heroin case. The 6-MAM identification secured the supplier's conviction under NDPS Section 21. The 2013 Bihar mid-day meal disaster at Dharmasati Gandaman primary school, where 23 children died after eating rice contaminated with monocrotophos, was confirmed at FSL Patna by identification of parent monocrotophos plus the dimethyl phosphate metabolite in stomach contents. Punjab and Rajasthan ethylene glycol suicides are confirmed every year at the regional FSL through glycolic acid identification and renal oxalate crystals on histopathology.

A complete SFSL report states the matrix sampled, the method (LC-MS/MS with multiple reaction monitoring), the internal standard (deuterated analogue), the limit of detection and cut-off, and an explicit post-mortem redistribution caveat. A report that says "morphine 480 ng/mL, 6-MAM 12 ng/mL, femoral blood, LC-MS/MS with morphine-d3 IS, LOD 1 ng/mL, identification of 6-MAM consistent with heroin intake within the hour preceding death" is admissible expert evidence under Section 293 BNSS. A report that says only "morphine detected" is useless.