Benefit of Complete Response in Multiple Myeloma Limited to High-Risk Subgroup Identified by Gene Expression Profiling

Complete remission (CR) has long been considered an essential first step toward curing systemic malignancies with chemotherapy. In multiple myeloma, response definitions have varied historically (1). In the context of autotransplant-supported high-dose therapy trials, achieving CR (absence of monoclonal protein in serum and urine on immunofixation analysis and normal bone marrow aspirate and biopsy) has been associated with superior survival (2–5); however, investigations conducted by the Southwest Oncology Group failed to link survival duration to the magnitude of response (1). Whereas early onset of CR after one or two induction chemotherapy cycles seems to be crucial to sustained remission in most systemic malignancies, the median time to CR in multiple myeloma is nearly 12 months, reaching a cumulative rate of ∼50% only at 3 years despite intensive treatment as practiced in total therapy 2 (TT2; ref. 6). Few predictors of CR have been identified, such as an IgA isotype, elevated serum concentrations of lactate dehydrogenase (LDH), and the presence of cytogenetic abnormalities (7). More recently, we identified high levels of serum-free light chain as being predictive of higher CR rates (8). Likely reflecting higher tumor proliferative activity and hence greater sensitivity to cytotoxic chemotherapy, these variables paradoxically have also been linked to shorter event-free survival (EFS) and overall survival (OS), probably as a consequence of rapid tumor regrowth in this setting (2, 6–8).

Some long-term multiple myeloma survivors never achieved CR, as reported by Fassas et al. (9) among patients treated at the University of Arkansas. Recent gene expression profiling GEP studies revealed that, in comparison with subjects with monoclonal gammopathy of undetermined significance (MGUS), patients with multiple myeloma and a MGUS-like signature enjoyed superior OS despite a significantly lower CR rate, compared with patients presenting with non-MGUS-like multiple myeloma (10). Similarly, in cases of multiple myeloma evolution from a documented MGUS or smoldering multiple myeloma phase, CR was significantly lower without affecting survival adversely (11). These findings indicate that the association between CR and survival does not apply to all patients with multiple myeloma.

A large population of 668 uniformly treated patients with multiple myeloma enrolled in TT2 offered the unique opportunity to examine the role of CR and timing of its onset as a surrogate for EFS and OS (6). With access to a comprehensive multiple myeloma data base including information on pretreatment standard prognostic factors (SPF) and GEP data of CD138-purified plasma cells available for 326 patients (12), we examined the role of CR for OS and EFS in the context of low-risk and high-risk features defined according to SPF and GEP data (13).

Eligibility. Between October 1998 and February 2004, 668 newly diagnosed patients with progressive or symptomatic multiple myeloma (≤75 years of age, no more than one cycle of prior therapy) were enrolled in a prospective randomized phase III trial, TT2, which evaluated whether the up-front addition of thalidomide would improve the frequency of CR and thereby prolong EFS and OS (6). Written informed consent had been obtained in keeping with institutional and National Cancer Institute guidelines. The protocol had been approved by the Institutional Review Board and the Food and Drug Administration and was monitored by a Data Safety and Monitoring Board, as required by the National Cancer Institute for phase III trials.

Treatment. Details of TT2 have previously been reported (6). Briefly, TT2 consisted of four treatment phases, using four induction multiagent chemotherapy cycles, melphalan-based tandem transplants, four cycles of consolidation chemotherapy, and IFN maintenance with high-dose dexamethasone pulsing during the first year. At registration, patients were randomly assigned to a control arm without thalidomide or to the experimental arm with thalidomide; thalidomide was applied throughout TT2 until relapse. Although associated with superior CR and EFS, the thalidomide arm did not impart superior OS, justifying our analysis of all patients regardless of treatment arm.

Laboratory evaluation. Multiple myeloma workup included analysis of serum and urine protein electrophoresis and quantitation of serum levels of immunoglobulins, β₂-microglobulin, and C-reactive protein. Bone marrow plasmacytosis was estimated on biopsy and aspirate samples. Metaphase cytogenetic analyses were also routinely done and typically 20 Giemsa-banded metaphases were evaluated. The diagnosis of a multiple myeloma–typical cytogenetic abnormality required the presence of structural abnormalities or hyperdiploidy in at least two metaphases, whereas hypodiploidy had to be present in at least three abnormal metaphases (14). Such studies were carried out at baseline and serially after initiation of therapy per protocol requirements to define response and relapse (see below). These serial examinations were done before each protocol phase and then semiannually. SPF data were available for 617 patients; 326 of these also had GEP information (see below).

GEP analysis. GEP was done as previously described (12). High-risk and low-risk categories of multiple myeloma were defined according to a recently reported 70-gene model (13).

Criteria for response and relapse. Of 668 patients enrolled in TT2, 651 began treatment and were assessed for response. CR was defined using European Bone Marrow Transplant criteria and did not require resolution of focal lesions present on magnetic resonance imaging (15, 16). Relapse from CR was defined by the reappearance of a monoclonal protein in serum or urine. Relapse from partial remission entailed an increase in serum M-protein level or urinary M-protein excretion by at least 50%.

Statistical analyses. Data are reported as of February 2007. The Kaplan-Meier method was used to estimate EFS and OS (17). EFS was defined from date of registration to the occurrence of death from any cause, disease progression, or relapse, or censored at the date of last contact. OS was defined from date of registration to the date of death from any cause or censored at the date of last contact. Cumulative incidence of response was determined using off-study or death events as a competing risks (18). The Cox regression method was used to examine multivariate prognostic factor models for EFS and OS with CR and second transplant as time-dependent covariates (19). Time-dependent interaction terms between CR and the strongest prognostic factor in each model were also included and tested to determine whether the CR effect differed by risk. Superimposable Kaplan-Meier plots of OS, observed in the two treatment arms of TT2 (when all randomized patients or subsets with GEP information were considered), justified a joint analysis regardless of treatment arm.

OS and EFS for all patients and the 326-patient subset with GEP data according to thalidomide randomization.Figure 1 depicts OS and EFS Kaplan-Meier plots for all 668 patients enrolled in the TT2 protocol as well as for the 326 patients with all SPF and GEP data, revealing that the patient subset was representative of the overall TT2 population. Whereas there was no difference between the study arms in terms of OS, EFS was superior among patients randomized to thalidomide in both patient sets.

Multivariate analyses of features associated with survival in the 326 patients with complete data. When examined only in the context only of SPF, OS was adversely affected by the presence of cytogenetic abnormalities and elevated serum levels of LDH and creatinine (Table 1A), whereas EFS was inferior in case of cytogenetic abnormalities and high levels of β₂-microglobulin and LDH (Table 1B). Achieving CR status was an independent beneficial event for both OS and EFS, whereas randomization to thalidomide prolonged EFS but not OS. Univariate analyses are depicted for OS and EFS in Supplementary Table S1.

In the context of additional information on GEP-derived risk designation, OS was adversely affected by cytogenetic abnormalities and high LDH and, importantly, by GEP-defined high risk pertaining to 13% of patients. The hazard ratio (HR) value was 5.74 (P < 0.001) and thus superseded HR values for all SPF markedly, which was also the case for EFS (see Table 1). A time-dependent interaction term was observed between CR and GEP-defined high risk for both OS and EFS, in that the risk reduction after CR was much greater in the high-risk group than in the remainder [OS: HR, 0.35 (P = 0.040); EFS: HR, 0.33 (P = 0.009); see Table 1]. Thus, the effect of CR on OS and EFS was different in the GEP-defined risk groups, and conversely, the effect of GEP-defined risk was different in CR and no-CR groups.

As shown in Fig. 2, in the low-risk group, OS was similar regardless whether or not CR was obtained (HR, 0.67; P = 0.128); in contrast, for the high-risk group, CR was beneficial for OS (HR, 0.23; P = 0.001). For patients not achieving CR, high risk implied inferior OS (HR, 5.74; P < 0.001); for patients achieving CR, GEP-defined high risk was marginally significant for reduced OS (HR, 2.01; P = 0.109). Interestingly, CR could not be predicted by GEP-defined risk groups. As depicted in Table 1, the timely application of a second transplant significantly benefited both OS and EFS in the context of GEP-defined risk designation.

Kaplan-Meier survival plots depicting the preferential benefit of CR in GEP-defined high-risk myeloma. To depict the role of CR in the context of GEP-defined risk groups, three landmarks (12, 18, and 24 months) were considered (Fig. 3). Whereas insignificant for the low-risk group (see Fig. 3A-C), CR at 18 and 24 mo was associated with superior OS in patients with high-risk multiple myeloma (see Figs. 3E and F). A favorable contribution of a second transplant was only noted in patients who had achieved CR status (Fig. 4). These figures support the multivariate analyses with time-dependent covariates as reported above, showing differential effects of CR, depending on GEP-defined risk categories, and a favorable effect of second transplant regardless of GEP-defined risk but especially when CR had already been induced by the first transplant.

Logistic regression analysis of SPFs associated with high-risk GEP. In an attempt to provide investigators without access to GEP information with potential SPF substitutes, we examined by logistic regression analysis the SPF most strongly associated with GEP-defined high-risk designation (Table 2). Results showed that, on multivariate analysis, the patients with high-risk GEP had significantly higher frequencies of cytogenetic abnormalities (HR, 5.12; P < 0.0001), LDH elevation (HR, 3.42; P = 0.0007), and low albumin levels (HR, 3.32; P = 0.0025).

To our knowledge, this is the first comprehensive analysis of the importance of CR for survival in the context of SPF and state-of-the-art molecular genetic data among newly diagnosed patients with multiple myeloma. In view of the delayed onset of CR in multiple myeloma even with intensive treatment as applied in TT2, a landmark technique is usually applied to portray its effect. This approach is characterized by the attrition of patients satisfying the requirement of having survived until the landmark. An alternative statistical method of examining CR (and second transplant) as a time-dependent variable in a Cox regression model permitted the inclusion of all patients. In the model with both SPF and GEP data, CR was an independent positive contributor to superior OS and EFS only for high-risk multiple myeloma (13).

Unlike traditional SPF with HR values not exceeding 2, the 70-gene model captured a truly high-risk subset with unprecedented high HR values in patients not achieving CR. In this setting, early onset of CR (reflecting marked sensitivity to the treatment used) is of critical importance to prolonged disease control and survival. We postulate that, as had been the case in non-Hodgkin lymphoma with the advent of doxorubicin (20), patients with high-risk multiple myeloma are more likely candidates for curative therapy, whereas those with low-risk multiple myeloma may require prolonged or repeated treatment to achieve a chronic disease status. In the case of MGUS-like multiple myeloma, high-dose therapy seems be incapable of eliminating dormant MGUS-precursor plasma cells so that a MGUS-like condition is reestablished, which explains the lower CR rate observed in this setting and yet a superior survival (10, 11). In the GEP-defined high-risk group, we have been piloting, instead of or before high-dose melphalan, rapidly cycled non–stem cell–toxic combination chemotherapy to maximize dose intensity and density (21), an approach that has yielded favorable outcome in patients with high-risk lymphoma (22).

Our observation that a second transplant benefits especially patients already in CR after the first transplant is at variance with results reported by the Intergroupe Francophone du Myelome in their IFM94 trial, showing such benefit only among patients not yet in CR at the time of the second transplant (23, 24). Thus, as in other malignancies, further tumor cytoreduction beyond the level of clinical CR seems to be beneficial. The persistence of magnetic resonance imaging–defined focal lesions for an average of 2 years beyond the onset of clinical CR (as examined here) before their eventual resolution (16) strongly suggests that these lesions, shown by fine needle aspiration to harbor monoclonal plasma cells, secrete minimal amounts of immunoglobulin or are entirely nonsecretory.

We conclude that (a) CR per se does not confer favorable outcome except in the small subgroup of 13% of patients with truly high-risk multiple myeloma that can thus far only be defined by GEP; (b) lack of CR is not detrimental in the majority of more than 80% of patients with good-risk multiple myeloma; and (c) CR needs to be validated as a surrogate end point for OS in new agent trials in the context of multiple myeloma genetic subtypes as presented here.

We thank our data managers and research nurses whose diligent work was critical to obtaining the data presented in this article.