To our knowledge, this is the first study to replicate the recent discovery of ITPA genetic variants that protect against RBV-induced HA in an independent cohort of patients from ViraHep-C. We provide the first description of a strong association between ITPase deficiency and Hb reduction over the entire 48-week course of pegIFN and RBV therapy for genotype 1 HCV. We also address the clinical implications of these findings by reporting the protective benefit of the ITPA variants against RBV dose reduction.
Almost one-third of patients (30%) were identified to have moderate to severe ITPase deficiency and to be relatively protected from the hemolytic toxicity of RBV. Patients with moderate to severe ITPase deficiency were least likely to become anemic. The effect was most evident early in treatment but persisted throughout. Further, this protective effect translated into fewer RBV dose reductions and greater cumulative RBV exposure. Indeed, the greater RBV dose reduction in the patients with normal ITPase activity likely explains the convergence of the Hb curves over time. The use of erythropoietin was not allowed by the ViraHep-C protocol, so the issue of whether these variants predicted need for growth factor support could not be directly addressed, but it is logical to expect that ITPase deficiency would be very relevant.
It was interesting that despite this strong protective effect against anemia, and less need for RBV dose reduction, no benefit in terms of treatment outcome was observed. There are a number of possible explanations. Firstly, it is the minor alleles that are protective and only a minority of patients are protected against anemia. Thus, we were most likely underpowered to observe an effect on SVR. Secondly, there was a high incidence of RBV dose reduction and discontinuation for reasons other than anemia in ViraHep-C, which may have obscured any effect. Thirdly, we are assuming that RBV dose reduction is the only mechanism whereby ITPA
variants might increase the rate of SVR. However, if these genetic variants affect RBV pharmacodynamics outside the erythrocyte, it may be that ITPase deficiency also reduces antiviral efficacy, despite protecting against anemia. This would be consistent with the fact that no increase in SVR rate was observed, despite maintenance of RBV dosing, and with the recent observation that early-onset anemia (likely to lead to dose reduction) was associated with an increased rate of SVR in a large study that permitted the use of erythropoietin.14
Finally, it has also been suggested that the impact of RBV dose reduction on the likelihood of SVR may be negligible, as long as RBV is not discontinued.15,16
Therefore, the relationship among anemia, ITPA
variants, and SVR is complicated, and there may be opposing mechanisms involved that now require further investigation.
ITPase deficiency causes the accumulation of inosine triphosphate within red blood cells and is generally believed to be a benign condition,4–7
although it has been associated with thiopurine toxicity.17,18
RBV is metabolized within the red cells to RBV triphosphate, and it is the accumulation of RBV triphosphate that is believed to lead to oxidative damage to the red cell membrane and erythrophagocytic extravascular destruction.19
Whether high levels of inosine triphosphate restrict the conversion and accumulation of RBV triphosphate within red blood cells is not known, and the mechanism whereby ITPase deficiency, or inosine triphosphate accumulation, protects against RBV hemolysis is yet to be resolved. Direct investigation of the relationship between red cell ITPase activity, using functional assays, and accumulation of RBV/RNB metabolites, and hemolysis, will be required.
There are many genetic variants that have been shown to influence the pharmacokinetics of drugs but that nevertheless fail to have any well-documented impact on clinical decision making, even at the level of dose reduction.20
In contrast, the ITPA
variants described here are shown to be strongly associated with differential sensitivity to RBV-induced anemia as well as the need for dose reduction during treatment, meaning that they join a small set of pharmacogenetic variants that have been documented to influence clinical management.21,22
This finding has the potential to inform clinical decision making. Patients with normal ITPase activity require particularly vigilant monitoring of Hb levels early in therapy. Indeed, it has been shown that a decrease in Hb of 1.5 g/dL during the first 2 weeks of treatment is predictive of the development of severe anemia.23
Early intervention in this group with RBV dose reduction and/or growth factor support may be indicated to maximize safety and minimize premature discontinuation of RBV. Conversely, the smaller subgroup of patients, protected from anemia, could be monitored less frequently. Prospective studies will be required to investigate the clinical utility, as well as the cost-effectiveness, of these approaches. Patients who are ITPase deficient may also be candidates for more aggressive RBV dose escalation strategies, because higher-dose RBV has been associated with higher rates of SVR.24
This will require evaluation in randomized studies. Separation of patient subgroups based on the predisposition to develop RBV-induced anemia may also allow clinicians to consider therapy despite comorbidities that were previously considered relative contraindications, such as renal impairment or mild to moderate coronary artery disease (where the risks of developing significant anemia and its effect on the underlying disease are of concern). Whether genetic testing can expand treatment options for these complex patients must be prospectively investigated. Finally, a number of the new HCV direct antiviral drugs that are currently under investigation in combination regimens with pegIFN and RBV have been associated with increased rate of anemia (telaprevir, boceprevir) and frequent need for erythropoietin use (boceprevir).25,26
A specific role for ITPase function in mediating the anemia associated with these direct antivirals will need to be explored. As a final note, functional testing of ITPase activity is possible and will need to be explored and compared with ITPA
genotyping for the prediction of anemia risk.
In conclusion, functional variants of the ITPA gene, associated with ITPase deficiency, are protective against RBV-induced hemolytic anemia, reducing the need for RBV dose reduction, and maintaining cumulative RBV dosage. ITPA genotype was not associated with SVR, although the sample size of this cohort may have been too small to detect a modest effect. ITPA genotyping, or functional ITPase assays, may help guide clinical decision making in patients considering antiviral therapy for HCV, especially those at high risk for hemolytic anemia or anemia-related morbidity.