Differences between unselected patients and participants in multiple myeloma clinical trials in US: a threat to external validity

External validity of clinical trials is affected by dissimilarities between study subjects and patient population. We identified 128 manuscripts (8,869 subjects) published between 2007 and 2014 reporting results of multiple myeloma (MM) trials performed entirely in the US. Characteristics of subjects were compared with unselected patients from SEER-18. Median of median age of subjects was 61 years vs. median age of unselected patients of 69 years. Trial subjects with untreated MM had less advanced stage than unselected patients. Racial-ethnic composition was informed in only 51 (39.8%) trials. Industry-sponsored trials were more likely to report accrual of minorities than National Cancer Institute (NCI) or investigator-sponsored trials. The observed/expected minority accrual was 0.52 (95% CI 0.49-0.55), being lower (0.43) in investigator-sponsored and higher (0.61) in industry-sponsored trials. We concluded that minorities, older individuals and persons with more advanced disease are underrepresented in MM trials, potentially compromising external validity of results.