My approach to treating SMM takes into account what its history can teach us about 1) how advancements in imaging and diagnostic reclassifications can revise the entire natural history of a disease, and 2) how evidence generated by even the best of studies may have an expiration date.
Huntsman Cancer Institute
Manni Mohyuddin, MD
Much of what we know about SMM today dates to a pivotal study by Robert A. Kyle, MD, and colleagues, published in 2007. That inspirational team of investigators followed people diagnosed with SMM from 1970 to 1995 and established the first natural history of the condition. Their monumental effort and the data and conclusions it generated (eg,10% risk annually of SMM becoming MM for the first 5 years) are still cited today in references, papers, and slide sets.
Despite the seminal importance of this work, from today’s perspective the 2007 study might just as well have been describing a different disease. Back then people were diagnosed with SMM if their blood work detected a monoclonal protein and a follow-up bone marrow biopsy found at least 10% plasma cells (or a monoclonal protein exceeding 3g/dL). If there were no signs of end-organ damage (ie, no anemia or kidney problems) and an x-ray showed no fractures or lesions in the bones, the diagnosis was determined to be SMM.
What’s different in 2024? First and foremost: advanced, highly sensitive imaging techniques. MRIs can pick up small lytic lesions (and even the precursor to lytic lesions) that would not appear on an x-ray. In fact, relying solely on x-rays risks missing half of the lytic lesions.
Therefore, using the same criteria, many people who in the past were diagnosed with SMM would today be diagnosed with MM. Furthermore, in 2014 a diagnostic change reclassified people’s diagnosis from the highest risk category of SMM to the category of active MM.
Due to these scientific advances and classification changes, I believe that the natural history of SMM is unknown. Risk stratification models for SMM derived from data sets of people who had not undergone rigorous advanced imaging likely are skewed by data from people who had MM. In addition, current risk stratification models have very poor concordance with each other. I routinely see people whose 2-year risk according to different models varies by more than 30%-40%.
All this information tells us that SMM today is more indolent than the SMM of the past. Paradoxically, however, our therapies keep getting more and more aggressive, exposing this vulnerable group of people to intense treatment regimens that they may not require. Therapies tested on people diagnosed with SMM include an aggressive three-drug regimen, autologous stem cell transplant, and 2 years of additional therapy, as well as, more recently CAR T-cell therapy which so far has at least a 4%-5% treatment-related mortality risk in people with myeloma and a strong signal for secondary cancer risk. Other trials are testing bispecific therapies such as talquetamab, a drug which in my experience causes horrendous skin toxicity, profound weight loss, and one’s nails to fall off.
Doctors routinely keep showing slides from Kyle’s pivotal work to describe the natural history of SMM and to justify the need for treatment, and trials continue to use outdated progression prediction models. In my opinion, as people with MM keep living longer and treatments for MM keep getting better, the threshold for intervening with asymptomatic, healthy people with SMM should be getting higher, not lower.
I strongly believe that the current landscape of SMM treatment exemplifies good intentions leading to bad outcomes. A routine blood test in a completely healthy person that finds elevated total protein in the blood could culminate in well-intentioned but aggressive therapies that can lead to many serious side effects. (I repeat: Secondary cancers and deaths from infections have all occurred in SMM trials.)
With no control arm, we simply don’t know how well these people might have fared without any therapy. For all we know, treatment may have shortened their lives due to complications up to and including death — all because of a blood test often conducted for reasons that have no evidentiary basis.
For example, plasma cell diseases are not linked to low bone density or auto-immune diseases, yet these labs are sent routinely as part of a workup for those conditions, leading to increasing anxiety and costs.
So, what is my approach? When treating people with SMM, I hold nuanced discussions of this data to help prioritize and reach informed decisions. After our honest conversation about the limitations of SMM models, older data, and the limitations of prospective data studying pharmacological treatment, almost no one signs up for treatment.
I want these people to stay safe, and I’m proud to be a part of a trial (SPOTLIGHT, NCT06212323) that aims to show prospectively that these people can be watched off treatment with monitoring via advanced imaging modalities.
In conclusion: SMM teaches us how, even in the absence of pharmacological interventions, the natural history of a disease can change over time, simply via better imaging techniques and changes in diagnostic classifications. Unfortunately, SMM also illustrates how good intentions can lead to harm.
Dr. Mohyuddin is assistant professor in the multiple myeloma program at the Huntsman Cancer Institute at the University of Utah in Salt Lake City.