Plasma Cell Acid Phosphatase in Multiple Myeloma

In a recent issue of Blood, Saeed et all reported about the prognostic significance of plasma cell acid phosphatase (PCAP) in a large number of patients with multiple myeloma, showing that a low PCAP score (< 130) at diagnosis might recognize a subgroup of myelomatous patients with poor prognosis. I believe that such findings are quite surprising in view of the previously published cumulative data on this topic, including personal experience: and of some biologic considerations. The same group had found a direct correlation between a high PCAP score and “active” phases of disease in selected myelomatous patients3; our studies had confirmed these observations.’ In addition, such as emerged also in the report in discussion, MGUS and “reactive” bone marrow plasmocytosis usually evidence lower PCAP activity than that measured in “overt” myeloma; overlaps may occur, but only at low levels of PCAP score. Finally, in multiple myeloma, a direct correlation between PCAP and plasma cell proliferative activity (one of the most important prognostic factors in this disease), has been r e ~ o r t e d . ~ . ~ All of these findings indicate a possible relationship between high levels of PCAP and intrinsic biologic “aggressiveness” of myeloma cells, a fact that seems to contrast with the conclusions of Saeed et al. Even when we have recently reevaluated PCAP in 102 cases of multiple myeloma adopting the modified scoring method proposed by these investigators, we obtained a good correlation between high PCAP score at diagnosis and poor prognosis (PCAP score >200 v 1200: median overall survival 18 v 59 months, respectively, P < ,001). In particular, increased levels of PCAP were detected in three cases of plasma cell leukemia with very short survival. As reported previously: we found that the only group of myelomatous subjects who showed a poor clinical outcome associated with a constantly low PCAP score was confined to those patients with a high percentage (>40%) of “flaming” plasma cells. We speculated that the large amount of glycoproteins that is present in these cells could mask the positivity of the cytochemical reaction or, in turn, that the wider cytoplasm extension could lead to a lower total score for PCAP. In this setting, it is noteworthy that the prognosis of multiple myeloma with “flaming” morphologic features has sometimes been reported to be unfavorable (and overall survival of such cases in our series was significantly lower than that of other patients). It would be of interest to know whether Saeed et a1 looked at this phenomenon. In conclusion, in view ofthe contrasting data at present time available on this topic, 1 believe that the clinical significance of PCAP in multiple myeloma, if any, remains unclear.