Hemoglobin liberated to plasma during intravascular hemolyses is rapidly bound to haptoglobin. The hemoglobin-haptoglobin complexes undergo endocytosis through the monocyte/macrophage specific scavenger receptor for hemoglobin (CD163). This mechanism protects against oxidative and NO-scavenging adverse effects of free hemoglobin. In this study, we describe a novel syndrome of severe intravascular hemolysis and serious hemolytic crisis due to impaired hemoglobin scavenging in three acute myeloid leukemia patients following CD33-directed therapy with the immunotoxin gemtuzumab ozogamicin (GO, Mylotarg™). A synchronous high free hemoglobin, haptoglobin, and low bilirubin after septicemia-induced intravascular hemolysis indicated abrogated clearance of haptoglobin-hemoglobin complexes. This was further supported by low levels of plasma soluble CD163 and a concordant low number of CD163-expressing monocytes. We show that CD163 positive monocytes and bone marrow macrophages coexpress CD33 thus suggesting that the GO-induced cellular cytotoxicity of CD33 positive cells eradicates a significant part of the CD163 positive monocytes and macrophages. The patients had severe inflammation and serious organ failure symptoms that may be a direct effect of the persistent high level of free hemoglobin. One of the patients had a fatal outcome whereas, the other two recovered from the hemolytic episode, and the peripheral blood CD163 expression returned to normal. The risk of a serious hemolytic crisis should be considered following CD33-targeted chemotherapy. Furthermore, the cases provide circumstantial evidence of a key role of CD163 plasma hemoglobin clearance in vivo.

Author notes

Disclosure:Research Funding: Danish Medical Research Council (22-03-0355, HJM).