Second malignancies in multiple myeloma; emerging patterns and future directions

Abstract

The changing landscape of treatment options for multiple myeloma has led to a higher proportion of patients achieving deep, long-lasting responses to therapy. With the associated improvement in overall survival, the development of subsequent second malignancies has become of increased significance. The risk of second malignancy after multiple myeloma is affected by a combination of patient-, disease- and therapy-related risk factors. This review discusses recent data refining our knowledge of these contributing factors, including current treatment modalities which increase risk (i.e. high-dose melphalan with autologous stem cell transplant and lenalidomide maintenance therapy). We highlight emerging data towards individualized risk- and response-adapted treatment strategies and discuss key areas requiring future research.

1. Introduction

The prognosis for patients with multiple myeloma (MM) has significantly improved over the past twenty years, which has been driven by the improved diagnostics, prognostication and efficacy of anti-MM treatment. The median overall survival (OS) for patients with favourable risk disease treated with modern therapies now exceeds 10 years, both in randomized clinical trials (1, 2), and in studies based on national cancer registries(3, 4). An improvement in survival has been observed in each of transplant-eligible(2) and -ineligible patients(5). Many recent studies have demonstrated that those patients achieving a very deep response to therapy, i.e. no detectable minimal residual disease (MRD(−)), may continue to enjoy a disease-free state for many years(6–11). Therefore, MM has an increased prevalence in the general population as patients are living longer before succumbing to myeloma or other disease(4). This extension of survival has reinvigorated debate on the significance of second malignancies following MM.

2. Defining Second Malignancy (SM) in MM patients

2a. Incidence of SM after MM

The incidence of SM in MM patients has been estimated from each of population-based registry studies, retrospective analyses and prospective clinical trials. These estimates vary due to the different populations studied and the change in treatment regimens over time. The overall rate of “any SM” are consistently low across studies, with 5–7% affected, and standardised incident rates (SIR; calculated from the ratio of the observed rate to the expected rate) of 0.98–1.26(12–14). However, variance is observed between sub-types of malignancy, with a significantly higher incidence of haematological cancer demonstrated compared with solid organ cancers(12–18). Population-based studies inclusive of patients treated prior to the wide-spread use of immunomodulatory agents (IMiDs) and proteasome inhibitors (PIs) documented SIR for myelodysplastic syndromes (MDS) / acute myeloid leukaemia (AML) of 6.5–11.5(13, 14, 16, 17), with increased rates also observed in acute lymphoblastic leukaemia (ALL, SIR 5.5), chronic myeloid leukaemia (CML, SIR 2.3) and non-Hodgkin lymphoma (NHL, SIR 1.3–1.7)(13, 16, 17).

These patterns have been replicated in more recent registry studies(12, 15, 19) and retrospective reviews(20), with a reduction in incidence noted as the use of prolonged upfront alkylating agents declines. In addition, since reports of an increase in secondary malignancies following the use of lenalidomide(21, 22), phase III clinical trials have routinely included this as a secondary outcome measure. A summary of results from major population-based registries and retrospective analyses are presented in the International Myeloma Working Group overview published in 2016(23), while malignancy following particular therapies for MM are discussed in section 5 below.

The incidence of subsequent non-haematological cancer in patients diagnosed with MM is much lower than haematological cancers, but particular entities do have an increased rate. Registry studies report increased rates of gastrointestinal (SIR 1.3–2.03), kidney (SIR 1.3–1.51), bladder (SIR 1.22), melanoma (SIR 1.36–1.43) and non-melanotic skin cancers (SIR 2.22) and while reduced incidence rates have been observed in lung (SIR 0.28–0.88), breast (SIR 0.76–0.81) and prostate cancer (SIR 0.69–0.75) (12–14, 17, 18, 24). It has been reported that breast and prostate cancers are more commonly diagnosed at an early stage in patients with MM(13, 25), which may reflect closer attention to cancer-surveillance practice in patients already suffering a first malignancy.

2b. Features of therapy-related myeloid neoplasms; overall and after MM

Cancers of the myeloid lineage following chemo- or radiotherapy are defined as therapy-related myeloid neoplasms (t-MN) according to the World Health Organisation classification(26). Comprising therapy-related MDS (t-MDS), AML (t-AML) and myelodysplastic / myeloproliferative overlap syndrome (t-MDS/MPN), these entities have different clinical and pathological features compared with de novo myeloid neoplasms (dn-MN)(27–29). T-MN account for 6–7% of AML cases(29–31), and up to 10% all MDS cases(32), with multiple analyses including those using the United States (US) Surveillance, Epidemiology and End Results (SEER) database documenting an increase in incidence over time, both overall and specific to patients with MM(28, 33).

Patients with t-MN tend to be diagnosed at a younger age compared with dn-MN, at a median of 64–65 years(28, 34). Multiple studies have demonstrated a higher incidence of clonal cytogenetic aberrations, in particular complex karyotypes and adverse features including hypodiploidy in the MN cells(29–31). Molecular analyses define a higher rate of TP53 mutations(35, 36) and a lower incidence of therapeutically targetable mutations (including IDH1/2, EZH2 and FLT3-ITD(37, 38)). The concept of clonal haematopoiesis as it relates to t-MN after MM is discussed in section 4 below.

2c. Features of solid SM after MM

Solid malignancies occurring after MM have some differences from the primary cancers affecting the general population which largely reflect MM epidemiology, including an older age of diagnosis, a male predominance, and a higher percentage of black ethnicity(24). However, many of the prognostic features remain the same, including rates of hormone-receptor positivity in breast cancer, prostate-specific antigen levels in prostate cancer, lymph node involvement in colorectal cancer and proportion of adenocarcinoma amongst lung cancer cases(24, 25).

2d. Outcomes from SM after MM

Overall, outcomes from t-MN are less favourable than with dn-MN(28, 33, 38, 39). It has been demonstrated that the MDS prognostic scoring system remains discriminatory of risk with t-MDS, however the distribution of cases is not equivalent, with a shift toward high risk in t-MDS and a worse survival within each risk category compared with dn-MDS(40). Therefore, t-MN have a more aggressive clinical pattern compared with dn-MN and are less responsive to treatment, resulting in a median OS estimates between 8 and 14 months(28, 34, 41).

Outcomes from non-haematological cancers are less dismal than with t-MN, with some studies reporting a subset of patients showing a similar response to therapy as in de novo disease, particularly those able to tolerate intensive treatment(25, 42). There is obviously heterogeneity when considering outcomes from all solid organ malignancies, and the pattern of an earlier stage of diagnosis will likely affect outcomes. A SEER-based analysis detailed a significantly higher overall mortality (when compared with patients not having MM) with almost all cancers studied (HR 1.84–2.81, excepting lung cancer), however the mortality due to the solid-organ malignancies were out-weighed by the MM-related death(25). The respective risk and effect on outcomes may change over time as MM survival improves.

3. Patient-related factors contributing to susceptibility to SM after MM

The development of SM after MM is multifactorial, which complicates the accurate estimation of risk for a particular patient or circumstance. However, several host-related factors appear to contribute to the risk of subsequent SM after MM, as detailed below, in Table 1 and in Figure 1.

Table 1.

Factors affecting risk for a second malignancy following a diagnosis of multiple myeloma

	Risk factor	References
Patient-related	Increasing age	11, 13, 18, 20, 23
	Male sex	13, 15, 18, 20
	Ethnicity	15, 41–44
	Genetics; single nucleotide polymorphisms, DNA damage repair, drug metabolism, CDKN2A mutation	36, 45–55
	Environment; smoking, obesity	18, 56–57, 60–65
	Comorbidities; prior cancer, autoimmune disease	66–67, 70–74
Disease-related	M-protein size and immunoglobulin subtype	12
	Clonal haematopoiesis	34, 75, 81
	Myeloma genetics	79–80
	Immune dysregulation	83
Treatment-related	Alkylating drugs	84–86
	Ionizing radiation	13, 87
	High-dose melphalan and autologous stem cell transplant	13, 86, 88–93
	Lenalidomide	20, 94–99
	Bortezomib	10, 102–105
Emerging areas for ongoing research	Myeloma genomics; rare structural variants, mutational signatures, defining the effect of clonal haematopoiesis	90, 116–120
	Tumour microenvironment	122–124
	Optimal timing for treatment initiation	125–128
	Role of autologous transplant	93
	Optimal duration and agent for maintenance	104–105
	Effect of novel therapies on second malignancy	130–132

Figure 1.

Established and emerging risk factors for the development of second malignancy after multiple myeloma

3a. Age

Increasing age is associated with increased risk of t-MN, with a recent analysis using the US SEER database demonstrating a hazard ratio (HR) of 2.5 (1.5–3.9) in patients ≥55 years compared with those ≤55 years old(15). Previous estimates from earlier (alkylator-rich) treatment periods demonstrated an increased incidence of t-AML in patients <65, with the hypothesis that younger patients may have received more intensive up-front treatment(13). More recent studies in transplant-eligible patients detail an increased rate of t-MN specifically and SM overall in patients having an advanced age at transplantation(15, 20).

Regarding solid organ cancers; those with preceding MM are older on average than those with de novo disease, reflecting the demographics of MM patients overall(25). Increasing age is a known risk factor for many malignancies, and several multivariate analyses have confirmed advanced age to increase risk for SM in patients having MM(20, 22). As more efficacious therapies continue to improve outcomes for MM, survival bias may affect analyses of the effect of age in SM, with an increasingly prolonged opportunity for second malignancies to develop, unrelated in etiology to the history of MM.

3b. Sex

A recent analysis of SEER-database data detailed a HR of 2.3 (1.4–3.5) for male compared with female sex in MM patients treated with high-dose melphalan and autologous stem cell transplant (HDM-ASCT) (15). This confirmed previous reports(17), and has been demonstrated specifically in the post-transplant setting (HR for female 0.35–0.71)(20) and in meta-analyses assessing lenalidomide maintenance (HR for male 1.01–1.88)(22).

3c. Ethnicity

Ethnicity have been described in multiple studies to be a risk factor for the development of both monoclonal gammopathy of uncertain significance (MGUS) and subsequent MM(43–45). SEER-based analyses have highlighted the impact of ethnicity on the risk of subsequent SM, with varying effect according to the sub-type of SM. Reports include Non-Hispanic white patients having an increased risk of melanoma, AML and non-Hodgkin lymphoma, Hispanic white patients having a reduced risk of lung and prostate cancer, African Americans patients having an increased risk of renal cancer, and Asian- Pacific Islanders having a higher risk of AML(17, 46).

3d. Genetics

Genome-wide association studies have located multiple loci associated with susceptibility to MGUS and MM(47–49). Genes annotated by affected single nucleotide polymorphisms may also be relevant to subsequent development of SM, particularly those affecting DNA-damage repair (DDR) mechanisms and drug metabolism(50–52). It has been reported that nearly 10% of the population has a reduced capacity to respond to DNA-damage(51), with germline mutations (i.e. in TP53, BRCA1, BRCA2, MSH6, and ATM) detected in a higher proportion than expected when somatic screening was performed in malignancy (53), potentially contributing to the genetic instability observed in some MM patients(54), with subsequent t-MN containing a 20% incidence of germline DDR-gene mutations(38, 55). In addition, germline mutations in the CDKN2A gene have been reported to predispose to both MM and additional cancers(56), and a polymorphism affecting the erythropoietin gene promotor is associated with differing rates of MDS following MM(57).

3e. Environment / Behaviour

Smoking has not been associated with an increased risk of MM(58, 59), nor an increased risk of SM following MM beyond that associated with individual de novo cancers(20). Moderate alcohol intake has been reported to potentially reduce the incidence of MM(60, 61), and there is no strong data to suggest an increase in SM related to alcohol in MM. Obesity has been described as a risk factor for both the development of MGUS(62) and for progression from MGUS to MM(63, 64). In addition, obesity has been shown to be associated with an increased rate of SM following MM(20), which is postulated may relate to obesity-associated chronic inflammation(65, 66), with obesity also being associated with adverse cytogenetics in t-MN(67).

3f. Comorbidities- prior cancer / autoimmune disease

A large registry study demonstrated that MM patients with a history of prior malignancy had a higher risk of developing a subsequent cancer (HR 1.23–1.65), with an associated increased risk of death (HR 1.1–1.26), which was compounded if there had been more than 1 prior malignancy(68). The authors hypothesized that prior malignancy may reflect underlying genetic susceptibility to developing MM, though note chemo- and radiotherapy treating earlier cancers may contribute(69), just as pesticide exposure is reported to increase risk(70, 71).

Patients with autoimmune conditions are at an increased risk of developing several cancers including MM, which is due to both underlying immune dysregulation and to the use of immunosuppressive treatment regimens(72, 73). Pre-existing autoimmune disease predicts for an increased risk of death with MM, and for additional malignancy subsequent to MM(74–76). Dysregulation of the immune system can be viewed as affecting all 3 of patient-, disease- and treatment-specific risk of SM.

4. Disease-related factors contributing to susceptibility to SPM

The risk of MDS / AML is increased in those with MGUS, independent of whether or not MGUS progresses to MM, supporting a treatment-independent effect. The risk varies with MGUS disease characteristics including immunoglobulin isotype, (increased risk only with IgG/IgA) and the size of the paraprotein, (increased risk with >1.5g/L, SIR 11.12), with MGUS patients assessed in a population-based registry having an 8-fold higher risk of AML/MDS (95% CI 5.40–11.43) (14).

In addition, there is increasing interest in the role of clonal haematopoiesis (CH) in the etiology of t-MN, with the hypothesis that treatment may induce the expansion of a pre-existing myeloid clone(36, 77). Retrospective examination of bone marrow in MGUS/MM patients obtained prior to the development of MDS/AML demonstrated that morphologic and immunophenotypic features typical for myelodysplastic syndromes are present in many patients prior to the receipt of treatment(78–80). Recent high-intensity sequencing of circulating cell-free DNA (cf-DNA) found that the majority of both patients with cancer and controls had cf-DNA mutations consistent with CH(81). The somatic genetic changes affect the subsequent risk of SM, with some reflecting CH (i.e. DNMT3A, TET2, ASXL1, TP53) while others are specific to the plasma cell genome(82, 83).

In addition, it has been shown that previously-treated cancer patients have a higher rate of CH in prior samples, with an enrichment demonstrated in DDR-gene mutations and data demonstrating that these mutations provided a selection advantage to the affected clones(84). In patients who progressed to t-MN, the clone present at diagnosis of CH defined that detected at diagnosis of t-MN. While we know that AML is preceded by CH, the exact impact of CH occurring concurrently with MM in defining the risk of SM is still unclear.

Immune dysregulation is a well-documented feature of MM, both as an integral part of untreated MM and as a result of therapy(85). MM cells have a symbiotic relationship with the tumour microenvironment (TME) and the hypothesis has been postulated that changes within the TME, (the immune components in particular), may predispose to subsequent SM(86, 87).

5. Treatment-related factors contributing to susceptibility to SPM

It is clear from historical studies that prolonged alkylator therapy significantly increases the risk of SM and should be avoided(88–90). Traditional anthracycline-containing combinations and prolonged oral melphalan are appropriately no longer in common use in the US due to toxicity concerns. Likewise, the relationship between high doses of ionizing radiation and SM has been clearly documented(15, 91), though data specific to MM is more limited. Here, we will focus on topical concerns related to therapies currently prescribed for MM.

5.1. High-dose melphalan- autologous stem cell transplant (HDM-ASCT)

Historical data examining the incidence of SM following HDM-ASCT was complicated by the prevalence of alkylator-containing induction regimens, with several studies finding that the effect of the induction contributed more to the risk of SM than the HDM-ASCT itself (90). However, recent multivariate analyses have shown an increase in t-MN specific to the use of HDM-ASCT(15, 92). Both the SEER database and the Center of International Blood and Marrow Transplant Research (CIBMTR) showed a higher relative risk of AML (10–50 times) and of MDS (100 times) than the background rate for patients with MM. However, the risk was higher in patients who received HDM-ASCT (CIBMTR data) compared to all MM patients (SEER data). The authors demonstrate that in the context of ASCT, the risk factors of age, male sex and number of prior lines of chemotherapy confer increased risk for subsequent MDS/AML, (HRs of 2.47 for age >55, 2.27 for male and 1.77 for >2 prior lines).

The mechanism for an increase in MDS/AML is likely directly related to the alkylating action of melphalan, (consistent with previous data following prolonged oral dosing), though the associated immunosuppression may also contribute. Historical data noted transient MDS-associated cytogenetic abnormalities following HDM-ASCT(93), with the time between melphalan-exposure and subsequent MDS/AML noted to be relatively short(14, 20), each of which support an etiological link. In addition, recent data on mutational signatures in MM, (whereby different processes contributing to cancer development are found to generate different combinations of mutation types(94)), defined a mutational signature in MM patients specific to melphalan exposure (95, 96). Further research is planned to define the clinical impact of these data.

In a measure of overall toxicity as assessed by patients themselves, patient-reported outcome studies show salvage ASCT in relapsed/refractory MM to have a greater deleterious impact on quality of life, with lower overall global health scores at day 100 post-ASCT, and higher side effects scores (both overall and pain-specific) persisting 6-months from ASCT (97). These findings are particularly important given that studies in the era of modern therapies have shown that upfront ASCT improves progression-free survival (PFS) but not necessarily OS(98). While undoubtedly a potent therapy, it may be that we are able to achieve equal measures of efficacy with a lower degree of toxicity, including incidence of SM(99).

5.2. Lenalidomide / other immunomodulatory agents (IMiDs)

A modest but statistically significant increase in SM with lenalidomide maintenance therapy has now been reported as a consistent finding across multiple large phase III trials, both when received after ASCT(100, 101), or continuing after induction therapy(102). Meta-analyses have demonstrated a HR of 2–4 for haematological SM when compared with placebo/observation, (95% CIs being 1.14–3.61 and 1.15–12.62 in 2 studies following ASCT), with HR of 1.1–1.7 for solid organ SM (95% CIs being 1.04–2.80 and 10.62–2.00) (22, 103). A significantly shorter time to development of SM was also demonstrated(103). Notably, when used as a component of upfront therapy, the increased risk of SM seen with lenalidomide was predominantly seen in those patients co-prescribed melphalan, but not in those receiving concomitant cyclophosphamide, nor those receiving just lenalidomide/dexamethasone since diagnosis(22). It appears that the wider treatment context (i.e. lenalidomide as one component of combination drug therapy, or ASCT followed by lenalidomide maintenance) may contribute to the overall risk of SM due to lenalidomide.

The mechanism/s by which lenalidomide increases the risk of SM are likely multifactorial. Lenalidomide efficacy requires the expression of cereblon, a component of the E3-ubiquitin ligase complex which is involved in DNA-damage repair(104). This may partially explain increased long-term toxicity observed with lenalidomide in combination with oral melphalan or HDM-ASCT. IMiDs also have effects on the TME and the immune system, including inhibiting cell-surface adhesion, reducing osteoclastic activity, reducing cytokines (including IL-6 and TNF-α), changing T-cell subsets and NK-cell activity(105). A combination of these factors may promote subsequent SM and may contribute to the incidence of haematological SM being significantly higher than non-haematological cancers.

The data with other IMiDs is less clear than with lenalidomide; much of the data with thalidomide is confounded by concurrent alkylator therapy but the trend to increased incidence of SM has been published to occur at a lower rate than with lenalidomide(106). The data with pomalidomide is less mature, but there is no early signal for a significant increase in SM(107).

Considering all available data on lenalidomide in MM, outcomes are consistent regarding a statistically significant increase in SM. However, the meta-analyses are equally clear that the survival benefit provided by lenalidomide, both PFS and OS, outweigh the risk of death related to SM(22, 103). Therefore, the current risk-benefit analysis for the majority of patients remains in favour of lenalidomide.

5.3. Bortezomib

The available data on the risk of SM with the PI bortezomib is reassuring, with the addition of bortezomib to lenalidomide-dexamethasone induction not being associated with an increase in SM (0.4% with and 0.8% without)(108). In fact, some studies have reported a lower rate of SM; 1% vs 4% at 6 years with the addition of bortezomib to thalidomide and dexamethasone(109), with a registry analysis documenting a HR of 0.24 in MM patients treated with bortezomib(12).

Bortezomib is also in use in some centers as maintenance therapy, with data showing an improvement in PFS in the high-risk cytogenetic subgroup, a trend to a lower rate of SM compared with no maintenance and no cases of haematological SM(110, 111). The mechanism by which a potential improvement in SM may occur with bortezomib use is not known.

5.4. Other drugs

All drugs currently in use or under investigation for MM include the incidence of SM as a clinical trial outcome. There have been no concerning signals for an increased incidence of SM in any of the landmark trials for the PI’s carfilzomib or ixazomib, nor the monoclonal antibodies daratumumab or elotuzumab. Monitoring is recommended to continue for agents following regulatory approval and commencement of off-trial routine treatment. Analysis of the data for chimeric antigen receptor T cells, bispecific antibodies, antibody drug conjugates and other novel therapies are currently limited by small numbers and short follow-up periods.

6. Future directions

Due to the growing population of patients living with MM, the rapidly evolving treatment options and the ever-increasing survival times, it is critical to carefully document the incidence, prevalence and risk factors for SM following MM. It is optimal to collect this data with a standardized definition (rate per persons at risk per year), with routine incorporation into clinical trial design. Both non- and haematological cancers should be reported, as should both invasive and noninvasive cancers, which likely represent different modes by which MM disease biology and treatment are contributing to subsequent SM development.

6a. Patient-related factors

Considering recent reports regarding inherited DNA damage repair (DDR) deficits predisposing to both MM and to subsequent SM(54, 55), we require better definition of patients at risk, with increased testing of germline samples in parallel with the cancer genome. As treatment options widen, with increasing rates of deep MRD(−) response from potent induction regimens such as carfilzomib-lenalidomide-dexamethasone(11, 112, 113), this data may sway physicians away from DNA-damaging agents in some patients, including contributing to the decision to forgo upfront consolidative ASCT(99). This risk-benefit analysis may be further influenced by the addition of daratumumab which reportedly increases both rates of MRD-negativity and PFS(114). While, for the majority of patients, the use of HDM-ASCT appears efficacious in prolonging PFS with manageable toxicity, it seems likely that in the face of sustained MRD(−) responses, the risk-benefit ratio (which includes the risk of SM), may argue against HDM-ASCT in a subset of patients.

6b. Disease-related factors

MM is known to be genetically heterogenous, with a large number of mutations occurring at a low frequency(115–117). Limiting analysis to the exome results in an inadequate description of MM genomic development. Recent studies involving whole genome sequencing(118), investigation of the role of rare structural variants(119), defining the contribution of various mutational signatures(120) and examining the clonal evolutionary trajectory over time (96, 121) are better defining the genomic landscape of MM. These features, critical to the progression from precursor disease to MM, are no doubt also central to the risk of subsequent SM. Considering the differing cytogenetic and molecular features of t-MN compared with dn-MN(27, 38, 122), we suggest integrating into clinical trial design the collection of DNA for comprehensive genomic analysis, both at baseline and following treatment, to better define those at increased risk for progression in either the MM clone or in clonal haematopoeisis as it relates to t-MN.

It has been previously demonstrated that MM growth and progression involves complex interactions within the tumour microenvironment (TME). It would seem likely that examining this relationship further may give insight into the subsequent development of SM after MM. Advances in analytic techniques have enabled definition of tumours and their transcriptome at single cell level, which in MM has revealed information regarding intra-tumoural heterogeneity, subclonal architecture, genomic progression and the potential for defining drug sensitivity(123–125). Extending this single-cell analysis to examine the TME in concert with the tumour cells will likely elucidate further mechanisms related to both the progression of disease and to the development of SM. Though too expensive and computationally intensive at this point in time to be incorporated as routine in clinical trials, serial examination of tumour cells and the TME at a single cell level would elucidate how this relationship changes in response to therapy, including which changes precede and predict subsequent SM. We anticipate research in this area to improve our understanding of SM following MM.

6c. Treatment-related factors

The data appear clear that particular MM treatments contribute to the risk of SM, with strong historical evidence linking prolonged oral alkylator therapy to increased risk, and lenalidomide use consistently associated with a modest but statistically significant increase in incidence. In the context of a demonstrated increase in PFS, (and in some studies, including a meta-analysis, OS), with lenalidomide, this risk has been accepted as the lesser evil compared with the ongoing potential for progressive disease and myeloma-associated death. This risk-benefit analysis needs continued evaluation, however, with rapid changes in clinical practice. IMWG guidelines broadened the definition of those requiring treatment initiation in 2014 and multiple groups are attempting to better define those patients with “smoldering” MM (SMM) but high risk of imminent progression, suggesting earlier initiation of therapy(126–128). A recent publication suggested lenalidomide now be considered standard-of-care for SMM, based on a study in which two-thirds of patients had intermediate or low risk SMM, with the potentially sub-optimal primary outcome of PFS(129). We now find ourselves faced with the prospect of otherwise healthy SMM patients being started on lenalidomide to delay disease progression without clear direction on the optimal duration of therapy, the effects of this single-agent therapy on subsequent genomic progression, or adequate information on the rate of SM expected from what may be very prolonged treatment. While the increased risk of SM from lenalidomide as a maintenance therapy appears modest, the same may not be true if this therapy is prescribed for a considerably longer duration.

HDM-ASCT is currently regarded as the standard-of-care (with caveats) based on trials demonstrating a prolongation of PFS compared with no HDM-ASCT (98). Again, this assertion requires ongoing evaluation in the context of more potent induction therapies. It has been demonstrated that the attainment of MRD-negativity is more important than the mode by which a patient achieves this state(6). In the context of induction regimens able to elicit sustained MRD-negativity(7, 11), which will likely be improved with quadruplet antibody-containing regimens(130), the subsequent role for both HDM-ASCT and prolonged maintenance therapy should be re-examined. It appears evident that the contribution of subsequent consolidation therapy to MM control, (with the associated risk of SM), may be reduced if the induction therapy alone is able to attain sustained MRD-negativity.

As a part of our ongoing examination of SM incidence and risk, it seems prudent to consider the potential for risk-adapted therapy, including monitored cessation of maintenance therapy. While lenalidomide until progression has been shown to have PFS (and in some studies OS) benefit(131, 132), this may not be required with more potent induction, and may not be adding to response in all patients. It is hypothesized that in patients achieving a sustained MRD-negative state for 2–3 years, continuing maintenance therapy may be adding financial and clinical toxicity, without providing additional efficacy. In order to test this hypothesis, ongoing and upcoming clinical trials are examining planned cessation of lenalidomide, (guided by MRD-status in some trials), analogous to trials ceasing tyrosine kinase inhibitors in chronic myeloid leukaemia patients having achieved a prolonged major molecular remission (NCT01863550, ENDURANCE; NCT04071457, DRAMMATIC trials).

Finally, continued examination of SM rates following the diagnosis of and treatment for MM must consider the effect of novel therapies. Monoclonal antibodies, in particular daratumumab, are regarded as one of the most promising therapeutic classes to emerge for MM in recent years. The addition to triplet induction regimens appears to increase the depth of response(130), and daratumumab is being tested as a potential addition to / alternative to lenalidomide in the setting of maintenance therapy (NCT03901963, AURIGA trial). The long-term toxicities of daratumumab are still under investigation, and while the overall safety signal suggests excellent tolerability, there is increasing evidence of a significant increase in infectious complications(133). There is the theoretical possibility that the effects on the natural killer cell population, for example, may contribute over time to the incidence of SM following MM(134, 135). In the context of any agent known to affect immune functioning, ongoing surveillance and detailed documentation of SM is essential.

7. Conclusion

While this is an exciting time to be a MM physician, with a steadily increasing armamentarium of efficacious therapies having acceptable short- and long-term toxicities, the risk of SM should remain a strong consideration when selecting therapies for individual patients. When considering all available information, it appears clear that the risk of death from SM is significantly lower than the risk of MM-related death, therefore the primary consideration should be optimal management of the MM. Currently available data does not support changing treatment decisions based purely on the risk of SM. However, there is increasing evidence that the combination of particular host, disease and treatment factors will increase the risk of SM for some individuals with certain treatments. Therefore, this risk should be integrated into the choice of regimen and should be considered when designing future clinical trials.

Practice points.

• The risk of second malignancy after multiple myeloma diagnosis is low but is increasingly important in the context of improved survival.

• Risk-adapted treatment strategies should consider patient-, disease- and treatment-related factors.

• The rates of second malignancies after multiple myeloma will likely change as our treatment regimens evolve, suggesting ongoing risk-benefit analyses are necessary

Research Agenda.

• Ongoing research includes the contribution of inherited defects in DNA-damage responses, clonal haematopoiesis and the tumour micro-environment to the development of second malignancies after multiple myeloma

• Serial comprehensive genomic analysis will better define disease- and treatment-related risk factors.

• Personalised risk- and response-adapted treatment regimens are in development for multiple myeloma and will likely affect the subsequent rate of second malignancies

Key abbreviations

AML

acute myeloid leukaemia

cf-DNA

cell free DNA

CH

clonal haematopoiesis

CIBMTR

Center of International Blood and Marrow Transplant Research

dn-MN

de novo myeloid neoplasm

HDM-ASCT

high-dose melphalan- autologous stem cell transplant

HR

hazard ratio

IMiDs

immunomodulatory agents

MGUS

monoclonal gammopathy of undetermined significance

MDS

myelodysplastic syndrome

MM

multiple myeloma

MRD

minimal residual disease

OS

overall survival

PIs

proteasome inhibitors

PFS

progression-free survival

SEER

Surveillance, Epidemiology and End Results database

SIR

standardised incident rate

SM

second malignancy

TME

tumour microenvironment

t-MN

therapy-related myeloid neoplasm