Outcomes of Plasma Cell Leukemia Patients in the Era of Next-Generation Novel Agents: A Single-Center Retrospective Cohort Study

Introduction

Primary plasma cell leukemia (pPCL) is a rare and aggressive disease comprising 1–5% of all plasma cell dyscrasias. Although historically pPCL has been defined by circulating plasma cells (CPCs) ≥ 20% and 2.0 × 10⁹/L, many series require only one of these two criteria for diagnosis. Over time there has been a pressing need to change the definition further in order to capture patients at an earlier stage of the disease. Recent studies have shown that multiple myeloma (MM) patients who have CPCs ≥ 5% but < 20% at diagnosis have similarly poor outcomes. These thresholds for defining pPCL have not been studied in the current era of novel agents. While bortezomib-based regimens have been shown to extend pPCL patient survival, studies with next-generation agents such as carfilzomib (CFZ), pomalidomide, and daratumumab (DARA) are still scarce.

Methods

We performed a single-center, retrospective study of patients who at diagnosis had pPCL (defined as CPCs ≥ 20% or 2.0 × 10⁹/L) or had MM with < 20% CPCs (MM-CPC). Patients were treated at our institution between 1/1/2000–7/17/2020. Overall response rates were determined according to International Myeloma Working Group criteria. Overall survival (OS) was defined as the length of time between initiation of first line therapy and death. Progression-free survival (PFS) was defined as the length of time between initiation of first line therapy and first progression. OS and PFS were compared using log-rank tests.

Results

Of the 54 patients identified, 38 had pPCL and 16 had MM-CPC. The median age at diagnosis of the pPCL and MM-CPC groups were 59.2 (range 43–94) and 59.8 years (range 29–79), respectively, with a similar percentage of females, 47.4% vs. 37.5%. Both groups were similarly distributed by year of diagnosis; 76.3% and 62.5% of pPCL and MM-CPC patients were diagnosed between 2012–2020, respectively. The pPCL cohort had median CPCs of 40% (15–98) while the MM-CPC cohort had median CPCs of 4% (1–15). Median CPCs at diagnosis was 3.36 (0.44–179) × 10⁹/L in the pPCL group compared to 0.33 (0.04–1.79) × 10⁹/L in the MM-CPC group. Most patients for both groups were ISS stage III at diagnosis (57.9% in pPCL cohort, 56.2% in MM-CPC cohort). R-ISS assessment was not feasible due to missing data. A greater proportion of patients in the pPCL group (44.7%) had complex cytogenetics at diagnosis compared to the MM-CPC group (25.0%).

All patients were exposed to at least one novel agent over the entire disease course. Of these patients, 50 (92.6%) were exposed to bortezomib, 45 (83.3%) to lenalidomide, 33 (61.1%) to CFZ, 22 (40.7%) to DARA, 13 (24.1%) to pomalidomide, and 4 (7.4%) to venetoclax. Twenty-four patients (44.4%) received an autologous stem cell transplant (ASCT). At the best response to treatment, 76.7%, 62.8% and 37.9% of patients achieved a partial response or greater to first, second and third line therapy, respectively.

The median OS of the pPCL and MM-CPC groups was 34.5 and 35.5 months (p = 0.97) (Figure 1A), while the median PFS was 13.9 and 10.9 months (p = 0.52) (Figure 1B), respectively. The median follow-up was 28.1 months. No differences were observed for patients with CPCs < 20% compared to patients with CPCs ≥ 20% (p = 0.98). There was trend towards a better survival for patients with CPCs < 2 × 10⁹/L compared to CPCs ≥ 2 × 10⁹/L (p = 0.35). Since we did not observe any meaningful difference in OS or PFS between the pPCL and MM-CPC groups, we combined these groups for further survival analysis.

Patients exposed to either DARA or CFZ (n = 37) had a median OS of 59.2 months, while patients exposed to neither drug (n = 17) had a median OS of 11.7 months (p = 0.02) (Figure 1C). ASCT was associated with a prolonged median OS (66.8 months vs. 17.2 months, p = 0.0001) (Figure 1D), while a complex karyotype at diagnosis was associated with a poorer median OS (17.4 months vs. 66.8 months, p = 0.01).

Conclusions

In the era of next-generation novel agents, overall and progression-free survival of pPCL and MM-CPC patients are similar. Patients exposed to DARA or CFZ have an improved survival compared to those who did not receive these drugs. ASCT is also associated with a superior survival over those who did not receive a transplant. Further studies are needed to evaluate the efficacy of these next-generation drugs in this patient population.