We report a relapse of BPDCN while on treatment with single-agent tagraxofusp (12uL/kg), with no identifiable expression of CD123 as evaluated by two different methods (IHC and flow cytometry) and two different antibody clones. BPDCN is frequently diagnosed based on the co-expression of CD123, TCL1, CD56, and CD4 on neoplastic cells in the absence of other lineage-defining markers like MPO, CD19, and CD3. Although the absence of CD123 expression in our case challenges a diagnosis of BPDCN and raises close differential, especially with acute undifferentiated leukemia. The clinical history, characteristic morphology, and co-expression of CD56 and TCL1 (strong and nuclear) in the absence of markers defining other lineages confirmed the diagnosis [16, 17]. Also, flow cytometric findings were concordant on both peripheral blood and bone marrow specimen collected two days apart. The lack of identifiable expression of CD123 may potentially be explained by downregulation or internalization of receptor components. Also, a potential block of CD123 (IL3 receptor) by tagraxofusp causing a failure of identification is theoretically possible. However, it has not yet been reported even during therapy.
Tagraxofusp, recombinant IL3 + truncated (catalytic and translocation domain of DT), acts via a two-step process. First involves IL3 mediated binding/ internalization to target the CD123 [IL-3(r)-alpha] positive tumor cells. The second step involves the cytoplasmic localization of the truncated DT to cause ADP phosphorylation of the histidine 715 on eEF2, causing a block of protein synthesis and eventually cell death. The resistance to tagraxofusp was evident in a subset of cases from early clinical trials. While no acquired somatic mutations were identified by whole exome sequencing, the resistance correlated with an acquired downregulation of DPH1 (diphthamide biosynthesis 1) in the tagraxofusp resistant clones [14, 15]. DPH1, the first enzyme in a series of at least seven member enzymes, converts histidine715 on eEF2 to diphthamide [18], resulting in loss of direct target of DT mediated inhibition of translocation, i.e., DT resistance.
Togami et al. elegantly describe the persistent expression of CD123 during and after treatment in BPDCN and AML to be associated with a survival advantage. Also, they highlight relative dependency on CD123 for survival in AML cell lines compared to other disease types, based on a gene-knockout experiment on cell lines from the genome-wide RNA interference screening [19] [20]. Thus, the absence of CD123 expression in myeloid lineage cancers, AML (and BPDCN) would be disadvantageous for the survival of the CD123 negative clone, especially during treatment.
Similarly, BPDCN resistance to tagraxofusp demonstrated by other authors has shown persistent surface expression of CD123. In their series of pediatric BPDCN, Sun et al. described tagraxofusp resistance in one case with multiple relapses with CD123 positive neoplastic cells after two rounds of therapy. Two other cases showed good initial response followed by disease progression. One of these (tested) cases was positive for CD123 at disease progression [12]. More recently, Samhouri et al. described a case of an elderly male with BPDCN (involving skin) and concomitant myelodysplastic syndrome with ring sideroblasts (bone marrow) as refractory to low dose tagraxofusp (5µg/kg; due to drug intolerance) and eventually achieved remission after switching to combination venetoclax and azacytidine[13].
Multidrug therapy has been used to overcome single-agent tagraxofusp resistance. Togami et al. confirmed the regulation of DPH1 by reversible promoter CpG DNA methylation, as seen previously [21], in AML and BPDCN. They showed reversal of DT-based resistance by synergistic cytotoxicity of CD123 mediated targeting by tagraxofusp with the hypomethylating effect of azacitidine in patient-derived xenografts models. Secondly, tagraxofusp resistance in both AML and BPDCN was associated with increased overall apoptotic priming and showed increased dependence on BCL-2 and or BCL-XL [15]. Such results have led to multiple trials evaluating combinations of tagraxofusp, azacitidine, and venetoclax (BCL-2 inhibitor) in BPDCN {NCT03113643, NCT03485547, and NCT04216524} Along similar lines, success stories of combination tagraxofusp and azacitidine in the management of relapsed BPDCN [22] highlight bright scope for specific multidrug management in BPDCN.