Association of TP53 (rs1042522) and BRCA1 Gene Polymorphisms with the Pharmacokinetics of 5-Fluorouracil among Breast Cancer Patients in Southern Punjab, Pakistan: A Step Towards Precision Oncology

Background Breast cancer is the most common malignancy among women in Pakistan accounting for about 40% of all female cancers, most of which present at advanced stage (II-III). 5-Fluorouracil (5-FU), a cornerstone cytotoxic agent administered as part of the FAC (5-FU, Adriamycin, Cyclophosphamide) regimen, exhibits up to 100-fold inter-individual pharmacokinetic (PK) variability, resulting in unpredictable therapeutic outcomes and dose-limiting toxicities. Germline polymorphisms in tumour suppressor genes specifically TP53 (rs1042522; Arg72Pro) and BRCA1 (exon 11) may modulate systemic drug exposure by influencing biotransformation enzyme expression, drug efflux transporter activity, and hepatic drug clearance capacity. Despite the clinical relevance of pharmacogenomics in optimising cytotoxic therapy, no study has investigated these associations in Pakistani breast cancer patients receiving 5-FU-based chemotherapy. Objective: To determine the allele and genotype frequencies of TP53 rs1042522 and BRCA1 exon 11 polymorphisms, and to evaluate their individual and combined associations with 5-FU pharmacokinetic parameters in 200 Pakistani breast cancer patients. Methods Two hundred histologically confirmed breast cancer patients (Stage II–III; n = 200) were prospectively enrolled from Nishtar Hospital, Multan. Genomic DNA extracted by the phenol-chloroform-isoamyl alcohol method was genotyped using validated Tetra-Primer ARMS-PCR. Plasma 5-FU concentrations at four pharmacokinetically informative time-points were quantified by validated RP-HPLC (Shimadzu LC-10AT; C18-ODS column; 254 nm). PK parameters including AUC₀₋ₜ, Cmax, t½, CL, Vd, Tmax, and MRT were computed using PK Solver software (one-compartment IV infusion model). Between-group comparisons used Kruskal-Wallis and Mann-Whitney U tests; independent predictors were identified by multivariable linear regression. Results TP53 genotype distribution was GG: 39 (19.5%), GC: 111 (55.5%), CC: 50 (25.0%); BRCA1 distribution was GG: 61 (30.5%), GA: 128 (64.0%), AA: 11 (5.5%). On Kruskal-Wallis analysis, neither TP53 nor BRCA1 genotype alone reached statistical significance for individual PK parameters. However, combined haplotype analysis revealed that BRCA1 AA-carrying haplotypes (GG + AA and CC + AA) had markedly elevated mean AUC₀₋ₜ (149.8 and 138.8 µg•h/mL respectively) and Cmax (77.9 and 59.2 µg/mL respectively) compared with non-AA haplotypes (AUC₀₋ₜ range: 38.9–71.2 µg•h/mL). Occupation and childhood residence were significant independent predictors of 5-FU exposure: housewife patients showed significantly higher AUC₀₋ₜ (128.3 vs 39.3 µg•h/mL; p = 0.001) and Cmax (61.8 vs 20.7 µg/mL; p = 0.002), and substantially lower clearance (17.5 vs 28.3 L/h; p = 0.003) compared with farming women. All 200 patients (100%) experienced alopecia; other adverse drug reactions varied by genotype with consistent trends. Conclusion TP53 and BRCA1 polymorphisms, particularly in combination, are associated with clinically meaningful differences in 5-FU pharmacokinetics in Pakistani breast cancer patients. Environmental and occupational xenobiotic exposure constitutes a significant additional PK modulator. Pre-treatment pharmacogenomic screening incorporating TP53 and BRCA1 genotyping, alongside established DPYD testing, may facilitate individualised dose adjustment, reduce toxicity burden, and advance precision oncology in South Asian patient populations.