Associations Between Polymorphisms of Genes Related to Vitamin D Pathway and the Response to Vedolizumab and Ustekinumab in Inflammatory Bowel Disease

<b>Background/Objectives</b>: Vitamin D (VD) has immunoregulatory properties, generating interest in its potential to influence therapeutic outcomes in inflammatory bowel disease (IBD), other than affecting the expression of genes encoding enzymes and transporters involved in drug metabolism and transport. This study investigated VD-related single nucleotide polymorphisms (SNPs) as predictors of clinical responses in patients with Crohn's disease (CD) and ulcerative colitis (UC) treated with vedolizumab (VDZ) or ustekinumab (UST) after 3 (T3) and 12 months (T12), as well as the achievement of fecal calprotectin (FC) levels < 250 mg/kg, a marker of mucosal healing. <b>Methods</b>: In this prospective study, 103 patients (67 CD, 36 UC) were enrolled, 40 receiving VDZ and 63 receiving UST. SNPs in the genes <i>CYP24A1</i>, GC, <i>CYP27B1</i>, and VD receptor (<i>VDR</i>) were analyzed via polymerase chain reaction (PCR) and associated with clinical and laboratory outcomes. <b>Results</b>: UST therapy demonstrated a higher clinical response rate at T12 compared to VDZ (<i>p</i> = 0.03). A correlation was found between response at T3 and T12 (<i>p</i> = 0.0002). GC 1296 AC polymorphism negatively predicted response at T12, with 63.6% of non-responders carrying this genotype. <i>CYP24A1</i> 8620 AG was a negative predictor for achieving FC < 250 mg/kg (<i>p</i> = 0.045). <i>CYP24A1</i> 22776 CT and <i>VDR</i> Cdx2 GG increased the likelihood of presenting CD over UC (OR 3.40, <i>p</i> = 0.009 and OR 3.74, <i>p</i> = 0.047, respectively). Additionally, <i>CYP27B1</i> -1260 GT and +2838 CT increased the likelihood of non-ileal CD (OR 3.13, <i>p</i> = 0.054; OR 7.02, <i>p</i> = 0.01). <b>Conclusions</b>: This study reveals associations between VD-SNPs, clinical response to VDZ and UST, and IBD phenotype and localization, supporting the development of personalized IBD treatment and warranting further validation.