Patients with the non-deletion genotype of HbH disease have higher proportions of HbH and more severe tissue hypoxia, than patients with the deletion genotype. As their red cells contain mainly two Hb species, HbH and HbA, one could exploit the high proportions of HbA by lowering its oxygen affinity and therefore increase the oxygen delivery of the red cells. Allosteric effectors which produce a low affinity Hb may be useful in this case. We investigated the effect of a bezafibrate derivative, RSR-4, on the oxygen affinity of red cells and purified hemolysates containing HbA and HbH. This allosteric effector crosses red cell membrane and binds reversibly to theα-chains of deoxy-Hb decreasing its oxygen affinity. The blood used was obtained from a patient with HbH disease (αTSaudi homozygote) whose HbH level was 33.5%, as measured by ce-HPLC. Oxygen binding studies were performed in red cells and purified hemolysates using the Hemox-Analyzer. The results showed that:

a. Red cells: at physiologic conditions 500μM RSR-4 results in an increase of P50 value from 14.5 to 35.2 mmHg. This decrease in the oxygen affinity was accompanied with a slight improvement of the cooperativity index n50 of 1.6 instead of 1.4.

b. Purified hemolysates: at 25 °C and pH 7.2 addition of 250μM or 500μM RSR-4 leads to a 7.5-fold or 11-fold decrease of the oxygen affinity respectively, while the natural allosteric effector 2,3-DPG achieves a 2.7-fold decrease.

In both cases, the oxygen equilibrium curves (OEC) are biphasic due to the presence of the non-cooperative, high oxygen affinity HbH (ß4) component. Upon addition of RSR-4, the lower part of the OEC (corresponding to HbH) is not shifted contrary to the upper part of the OEC (corresponding to HbA). These results confirm that RSR-4 does not bind to HbH.

As RSR-4 molecules cross the red cell membrane in spite of the presence of serum albumin, our findings provide an experimental model for lowering the oxygen affinity of HbH containing cells and suggest that tissue oxygenation in HbH disease could be ameliorated.