Patients with normal TPMT level received a higher starting dose of AZA than in patients who were heterozygous for TPMT deficiency (1.7 vs 0.9 mg/[kg x d], P < 0.0001).
Of 23 patients evaluated, six had TPMT deficiency (activity < 5 U/mL of packed RBCs [pRBCs]; homozygous mutant), nine had intermediate TPMT activity (5 to 13 U/mL of pRBCs; heterozygotes), and eight had high TPMT activity (> 13.5 U/mL of pRBCs; homozygous wildtype).
The development of severe bone marrow toxicity, in patients taking standard doses of thiopurine drugs, is associated with TPMT deficiency whilst the TPMT heterozygote is at an increased risk of developing myelosuppression.
In conclusion, clinicians should be aware of the impact of TPMT deficiency on the metabolism of thioguanine and should consider performing preemptive TPMT genotyping in combination with frequent blood test monitoring when using thiopurines in general.
The mutant alleles TPMT*2 (238G>C), TPMT*3A (460G>A, 719A>G), TPMT*3B (460G>A), and TPMT*3C (719A>G) account for 80-95% of TPMT deficiency observed in Caucasian populations.
The TPMT biochip was used to detect 6 TPMT single nucleotide polymorphisms (SNPs) corresponding to 7 TPMT-deficiency alleles (TPMT*2, TPMT*3A, TPMT*3B, TPMT*3C, TPMT*3D, TPMT*7, and TPMT*8).
To study the significance of TPMT deficiency in thiopurine metabolism and immunosuppressive activity in vitro, we established RNA interference-based TPMT knockdown (kd) in a Jurkat cell line.
The effects of genotyping or phenotyping a population for thiopurine methyltransferase (TPMT) status were compared using the prevalence of TPMT deficiency in Caucasians, the relative risks of neutropenia and the associated costs.
The specially designed TPMT biochip can recognize six point mutations in the TPMT gene and seven corresponding alleles associated with TPMT deficiency: TPMT*2; TPMT*3A, TPMT*3B, TPMT*3C, TPMT*3D, TPMT*7, and TPMT*8.
Three main TPMT alleles: TPMT*2 (c.238G>C), TPMT*3A (c.460G>A, c.719A>G) and TPMT*3C (c.719A>G) account for 80-95 % of inherited TPMT deficiency in different populations in the world.
Pharmacogenetic screening for TPMT polymorphisms before commencing azathioprine therapy may help to prevent severe hematotoxicity in patients with TPMT deficiency.
TPMT deficiency in one patient had led to pancytopenia whereas only two of the remaining four patients with hematotoxicity displayed an intermediate phenotype of TPMT.
This new LC-MS/MS is therefore a favourable clinical routine application to test TPMT activity, as it shows excellent performance in identifying patients with TPMT deficiency.
Upon mercaptopurine treatment, Tpmt <sup>-/-</sup> Mrp4 <sup>-/-</sup> mice had the highest concentration of bone marrow thioguanine nucleotides (8.5 pmol/5 × 10<sup>6</sup> cells, P = 7.8 × 10<sup>-4</sup> compared with 2.7 pmol/5 × 10<sup>6</sup> cells in wild-types), followed by those with Mrp4 or Tpmt deficiency alone (6.1 and 4.3 pmol/5 × 10<sup>6</sup> cells, respectively).
There is increasing evidence that testing for NUDT15 and TPMT deficiency in all populations prior to the start of thiopurine therapy is clinically useful and should be the first step in personalising thiopurine therapy.