Preemptive TPMT Genotyping and Adherence to Genotype-Based Therapeutic Recommendations Reduces the Healthcare Cost in Patients Receiving Azathioprine or 6-Mercaptopurine for Autoimmune Diseases

A cost analysis of thiopurine treatment was carried out in 257 patients, with 153 preemptively genotyped for TPMT and 104 retrospectively genotyped in a Spanish setting. The healthcare cost was significantly higher in patients retrospectively genotyped compared to those who were preemptively genotyped (p < 0.001). TPMT intermediate metabolizers (IMs) (n = 23) showed a 3.3-fold higher healthcare cost when compared to normal metabolizers (NMs) (p < 0.001). The healthcare cost in patients with a TPMT IM phenotype whose physician adhered to the genotype-informed recommendation was similar than the cost in TPMT NMs and was significantly lower than IMs whose physician did not adhere to the therapeutic recommendation (3.8-fold, p = 0.016). Myelotoxicity occurrence was significantly lower in patients preemptively vs. retrospectively genotyped (2.0% and 21.2%, respectively, p < 0.001). Patients who developed myelotoxicity showed a significantly higher healthcare cost than those who did not (4.10-fold, p < 0.001). Overall, 87% of patients whose dose was not adjusted despite being TPMT IMs suffered myelotoxicity, while only one of the eight patients (13%) whose dose was adjusted suffered myelotoxicity (p < 0.001). In conclusion, TPMT preemptive genotyping and physician adherence to genotype-informed therapeutic recommendations prevents myelotoxicity and significantly reduces the healthcare cost, and it is therefore essential for the sustainability of the Spanish healthcare system.