UGT1A1 and SLCO1B1 Pharmacogenetics

UGT1A1 and SLCO1B1 pharmacogenetics examines the molecular genetics and clinical prescribing guidelines for the premier Phase II conjugating enzyme pharmacogene (UGT1A1 on chromosome 2q37.1) and the premier hepatic uptake membrane drug transporter pharmacogene (SLCO1B1 on chromosome 12p12.1). Whereas cytochrome P450 enzymes mediate oxidative Phase I drug metabolism, UDP-glucuronosyltransferase 1A1 (UGT1A1) catalyzes the covalent attachment of glucuronic acid to lipophilic endobiotics (unconjugated bilirubin) and active anticancer metabolites (such as SN-38, the active metabolite of irinotecan and sacituzumab govitecan), while Organic Anion Transporting Polypeptide 1B1 (OATP1B1, encoded by SLCO1B1) mediates the active sinusoidal uptake of HMG-CoA reductase inhibitors (statins) and bilirubin conjugates from portal blood into human hepatocytes.

Inherited loss-of-function or decreased-function alleles in UGT1A1 (notably the TATA-box promoter repeat *UGT1A1\*28* and the East Asian coding variant *UGT1A1\*6) cause benign Gilbert syndrome at baseline but precipitate life-threatening neutropenia and late-onset secretory diarrhea upon exposure to irinotecan-based chemotherapy. Analogously, the decreased-function transporter allele SLCO1B1\*5* (c.521T>C, p.Val174Ala) impairs hepatic clearance of statins, elevating systemic skeletal-muscle exposure and driving statin-associated musculoskeletal symptoms (SAMS), myopathy, and rhabdomyolysis.

Alternative First-Exon Splicing of the UGT1A Locus and Gilbert Syndrome

The human UGT1A gene complex spans ~200 kb on chromosome 2q37.1 and employs a specialized exon-sharing genomic architecture: the 5' region contains nine functional, tandemly arrayed variable first exons (exons 1A1, 1A3, 1A4, 1A5, 1A6, 1A7, 1A8, 1A9, and 1A10), each driven by its own tissue-specific proximal promoter and TATA box, which are spliced via alternative cis-splicing onto a shared set of common exons 2 through 5 at the 3' end. Consequently, all nine UGT1A isoenzymes possess an identical C-terminal domain (binding the common cosubstrate uridine 5'-diphosphoglucuronic acid [UDPGA]) paired with a unique N-terminal aglycone substrate-binding domain encoded by each specific exon 1.

In the endoplasmic reticulum of hepatocytes and intestinal enterocytes, UGT1A1 is the sole physiologically relevant enzyme capable of conjugating hydrophobic, neurotoxic unconjugated bilirubin into water-soluble bilirubin monoglucuronide and diglucuronide for biliary excretion via the canalicular pump MRP2 (ABCC2). Complete biallelic loss of UGT1A1 activity causes fatal neonatal kernicterus (Crigler–Najjar syndrome type I), whereas marked reduction (5–10% residual activity) causes Crigler–Najjar syndrome type II (responsive to phenobarbital induction via CAR/PXR nuclear receptors).

In contrast, mild inherited reduction of UGT1A1 expression (~30% of normal hepatic activity) causes Gilbert syndrome—a benign, fluctuating, mild unconjugated hyperbilirubinemia (serum bilirubin 1.2–3.0 mg/dL, exacerbated by fasting or intercurrent illness) present in 5% to 12% of the global population. Two polymorphic alleles account for the vast majority of Gilbert syndrome and UGT1A1 Poor Metabolizer phenotypes:

1. *UGT1A1\*28* (rs3064744 / rs8175347): A microsatellite dinucleotide repeat polymorphism in the proximal promoter TATA box of exon 1A1. Whereas the wild-type *UGT1A1\*1* allele contains six TA repeats—A(TA)₆TAA—the decreased-function *UGT1A1\*28* allele carries an insertion of one extra TA dinucleotide—A(TA)₇TAA (while the rarer *\*36* allele has five repeats, A(TA)₅TAA [increased transcription], and *\*37* has eight repeats, A(TA)₈TAA [markedly decreased transcription]). Elongation of the TATA box from (TA)6 to (TA)7 reduces the binding affinity of the transcription factor TFIID / TATA-binding protein (TBP), lowering hepatic UGT1A1 mRNA transcription by ~60% to 70%. The allele frequency of *UGT1A1\*28* is ~30% to 40% in Europeans, Africans, and South Asians, and ~10% to 15% in East Asians.
2. *UGT1A1\*6* (c.211G>A, p.Gly71Arg, rs4148323): A non-synonymous missense mutation in exon 1 that reduces UGT1A1 catalytic efficiency by ~50% to 70%. Virtually absent in Europeans and Africans, *UGT1A1\*6* is prevalent in East Asian populations (allele frequency ~15% to 22%), where *\*6/\*6* homozygosity and *\*6/\*28* compound heterozygosity are major drivers of neonatal jaundice and drug toxicity.

Irinotecan (SN-38), Sacituzumab Govitecan, and Atazanavir Toxicity

1. Irinotecan (CPT-11) and SN-38: Irinotecan is a camptothecin-derivative prodrug used in metastatic colorectal, pancreatic (FOLFIRINOX), and small-cell lung cancers. Hepatic and systemic carboxylesterases (CES1 and CES2) cleave the bulky piperidinopiperidine carbamate side chain of irinotecan to generate its active metabolite SN-38 (7-ethyl-10-hydroxycamptothecin), which is 100- to 1,000-fold more potent as an inhibitor of DNA topoisomerase I than parent irinotecan. SN-38 is inactivated via glucuronidation by UGT1A1 into inactive SN-38 glucuronide (SN-38G), which is excreted into bile. Within the intestinal lumen, bacterial β-glucuronidases produced by gut microbiota deconjugate SN-38G back into free cytotoxic SN-38, which directly damages colonic crypt enterocytes unless re-glucuronidated by mucosal UGT1A1.

• Patients who are UGT1A1 Poor Metabolizers (*\*28/\*28*, *\*6/\*6*, or *\*6/\*28*) exhibit a ~40% to 60% reduction in SN-38G/SN-38 glucuronidation ratios and face a 2- to 4-fold higher risk of grade 3/4 neutropenia and late-onset secretory diarrhea (especially at irinotecan doses ≥180–250 mg/m²). FDA labeling and DPWG guidelines recommend reducing the initial dose of irinotecan by ~30% in known *UGT1A1\*28/\*28* or *\*6/\*6* homozygotes.

1. Sacituzumab Govitecan and Belinostat: Sacituzumab govitecan is a Trop-2-directed antibody-drug conjugate (ADC) carrying an SN-38 payload (drug-to-antibody ratio ~7.6); patients homozygous for *UGT1A1\*28* experience markedly higher rates of febrile neutropenia (~60% grade ≥3 neutropenia vs. ~35% in *\*1/\*1*). Similarly, the histone deacetylase inhibitor belinostat is cleared by UGT1A1 and requires a reduced starting dose (750 mg/m² vs. 1,000 mg/m²) in *\*28/\*28* homozygotes.
2. Atazanavir and Pazopanib: The HIV-1 protease inhibitor atazanavir and the tyrosine kinase inhibitor pazopanib competitively inhibit UGT1A1. In UGT1A1 PMs (*\*28/\*28*, *\*6/\*6*, or *\*6/\*28*), atazanavir precipitates clinically overt ocular scleral icterus and jaundice (total bilirubin >2.5–5.0 mg/dL) in 50% to 80% of patients, leading to premature drug discontinuation; CPIC recommends selecting an alternative antiretroviral agent (such as darunavir or dolutegravir) in UGT1A1 PMs.

Sinusoidal Hepatocyte Uptake and the c.521T>C (p.Val174Ala) Polymorphism

The SLCO1B1 gene on chromosome 12p12.1 encodes Organic Anion Transporting Polypeptide 1B1 (OATP1B1), a 691-amino-acid, 12-transmembrane-helix influx transporter localized specifically to the basolateral (sinusoidal) plasma membrane of human hepatocytes. Because HMG-CoA reductase inhibitors (statins) exert both their therapeutic pharmacological action (inhibiting hepatic de novo cholesterol synthesis and upregulating LDL receptors) and their metabolic/biliary elimination inside the hepatocyte, active transport by OATP1B1 is the rate-limiting step that clears statins from systemic arterial blood into the liver.

The canonical decreased-function variant in SLCO1B1 is c.521T>C (p.Val174Ala, rs4149056), located in transmembrane helix 4, which defines the *SLCO1B1\*5* allele (c.521T>C alone) and the *SLCO1B1\*15* haplotype (c.388A>G + c.521T>C). The Val174Ala substitution alters membrane helix packing, impairs post-translational N-glycosylation and sorting of OATP1B1 to the sinusoidal cell surface, and reduces intrinsic transport velocity (V_max).

When OATP1B1 function is impaired in c.521TC heterozygotes (Decreased Function) or c.521CC homozygotes (Poor Function, ~2–3% of Europeans and East Asians), statins are trapped in the systemic circulation rather than entering hepatocytes. Single-dose pharmacokinetic studies in c.521CC homozygotes versus c.521TT wild-type individuals reveal a hierarchy of systemic plasma exposure (area under the concentration–time curve, AUC) increases across different statins:

• Simvastatin acid (active open-ring hydroxy acid): +221% increase in plasma AUC (3.2-fold higher systemic muscle exposure).
• Pitavastatin: +162% to +191% increase in plasma AUC.
• Atorvastatin: +144% increase in plasma AUC (2.4-fold higher).
• Pravastatin: +90% to +100% increase in plasma AUC.
• Rosuvastatin: +65% to +87% increase in plasma AUC (additionally influenced by the efflux pump ABCG2 [BCRP] c.421C>A variant).
• Fluvastatin: No significant change (+19%), because lipophilic fluvastatin enters hepatocytes predominantly via passive transcellular diffusion and CYP2C9 metabolism rather than OATP1B1 transport.

The SEARCH Trial and 2022 CPIC Statin Prescribing Guidelines

In 2008, the SEARCH Collaborative Group published a landmark genome-wide association study in the New England Journal of Medicine examining patients taking 80 mg/day of simvastatin. Across the entire human genome, a single peak in SLCO1B1 (rs4149056, c.521T>C) reached genome-wide significance (p = 4 × 10−9), conferring an odds ratio for definite or incipient myopathy of 4.5 per copy of the C allele and 16.9 in CC homozygotes compared with TT homozygotes (cumulative 5-year myopathy incidence of 18% in CC vs. 0.6% in TT). Elevated systemic statin concentrations enter skeletal myofibers (via OATP2B1), where they deplete intramuscular ubiquinone (coenzyme Q₁₀) and geranylgeranyl pyrophosphate intermediates required for small GTPase prenylation, triggering statin-associated musculoskeletal symptoms (SAMS), creatine kinase (CK) elevation, necrotizing myopathy, and potentially fatal rhabdomyolysis with myoglobinuric acute kidney injury.

The updated 2022 CPIC Guideline for Statins integrates SLCO1B1, ABCG2 (for rosuvastatin), and CYP2C9 (for fluvastatin) phenotypes: in SLCO1B1 Poor Function (c.521CC) patients, simvastatin is contraindicated, atorvastatin starting doses are capped at ≤20 mg/day, and rosuvastatin or pravastatin starting doses are capped at ≤20–40 mg/day depending on concomitant ABCG2 status.

How does Gilbert syndrome protect against none of the toxicities of irinotecan while remaining benign without drugs?

At baseline without drugs, individuals with Gilbert syndrome (*UGT1A1\*28/\*28* or *\*6/\*6*) retain ~30% of normal hepatic UGT1A1 activity, which is sufficient to prevent unconjugated bilirubin from reaching the >20 mg/dL neurotoxic threshold that causes kernicterus, resulting only in harmless mild fluctuating jaundice (1.2–3.0 mg/dL). However, when challenged with high-dose irinotecan, this 70% reduction in UGT1A1 capacity creates a metabolic bottleneck for SN-38 detoxification, causing severe bone marrow neutropenia and intestinal crypt apoptosis.

Why does SLCO1B1 c.521T>C increase systemic statin toxicity while simultaneously slightly blunting LDL-cholesterol lowering?

Because OATP1B1 transports statins from the blood into the hepatocyte, a loss-of-function SLCO1B1 variant (c.521T>C) produces opposing effects in two compartments: outside the liver (in systemic arterial blood and skeletal muscle), statin concentrations rise 2- to 3-fold, driving myopathy; inside the hepatocyte (where HMG-CoA reductase resides), intracellular statin uptake is modestly slowed, slightly attenuating the per-milligram reduction in LDL cholesterol.

Why is fluvastatin largely unaffected by SLCO1B1 polymorphisms?

Unlike hydrophilic statins (rosuvastatin, pravastatin) and lactone prodrugs converted to organic acids (simvastatin acid, lovastatin acid) that rely heavily on active carrier-mediated transport via OATP1B1 to cross the sinusoidal hepatocyte membrane, fluvastatin is highly lipophilic at physiological pH, crosses hepatocyte membranes rapidly via passive diffusion, and is cleared primarily (>75%) by CYP2C9 oxidation rather than OATP1B1 uptake.

1. SEARCH Collaborative Group; Link, E., Parish, S., Armitage, J., Bowman, L., Heath, S., Matsuda, F., et al. (2008). "*SLCO1B1* variants and statin-induced myopathy—a genomewide study." New England Journal of Medicine, 359(8), 789–799. https://doi.org/10.1056/NEJMoa0801936
2. Cooper-DeHoff, R. M., Niemi, M., Ramsey, L. B., Luzum, J. A., Tarkiainen, E. K., Straka, R. J., et al. (2022). "The Clinical Pharmacogenetics Implementation Consortium guideline for SLCO1B1, ABCG2, and CYP2C9 genotypes and statin-associated musculoskeletal symptoms." Clinical Pharmacology & Therapeutics, 111(5), 1007–1021. https://doi.org/10.1002/cpt.2557
3. Gammal, R. S., Court, M. H., Haidar, C. E., Iwuchukwu, O. F., Gaur, A. H., Alvarellos, M., et al. (2016). "Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline for UGT1A1 and atazanavir prescribing." Clinical Pharmacology & Therapeutics, 99(4), 363–369. https://doi.org/10.1002/cpt.269
4. Innocenti, F., Undevia, S. D., Iyer, L., Chen, P. X., Das, S., Kocherginsky, M., et al. (2004). "Genetic variants in the UDP-glucuronosyltransferase 1A1 gene predict the risk of severe neutropenia of irinotecan." Journal of Clinical Oncology, 22(8), 1382–1388. https://doi.org/10.1200/JCO.2004.07.173