TERTIARY OBJECTIVES: I. To characterize the relationship between serum concentration of the active leflunomide metabolite, teriflunomide (A77 1726), and toxicity. (Phase I/II) II. To assess the relationship between serum concentration of the active leflunomide metabolite, teriflunomide (A77 1726), and disease response. (Phase I/II) III. To explore the relationship between polymorphisms in the CYP1A2, CYP2C19, or DHODH genes and toxicity/response. (Phase I/II) IV. To explore the ex vivo cytotoxicity of leflunomide toward primary multiple myeloma (MM) cells, in order to evaluate whether individual ex vivo leflunomide response might be a useful predictor of therapeutic response. (Phase I/II) V. To explore the potential additive or synergistic effects of combining leflunomide with other classes of Food and Drug Administration (FDA)-approved drugs. (Phase I/II) VI. To generate a preliminary ribonucleic acid (RNA)/microRNA (miRNA) and deoxyribonucleic acid (DNA) methylation signature associated with response of MM cells to leflunomide in vivo (mRNA/miRNA and DNA methylation, phase II only) and teriflunomide ex vivo (messenger RNA [mRNA]/miRNA). (Phase I/II)