This study is composed of three dosing plan stages. The initial stage of this trial is the dose-escalation stage. A standard dose-escalation 3+3 design will be utilized to identify a safe maximum tolerated dose (MTD) of STI-6129 in patients with relapsed or refractory systemic AL amyloidosis. After identification of the MTD, or the finding that the last dosing cohort is tolerated well (i.e., the maximum practical dose [MPD]), 12 patients will be enrolled to receive STI-6129 treatment at the MTD/MPD level to collect pharmacokinetic data ( the pharmacokinetic (PK) stage) to model a treatment schedule that achieves a stable effective serum concentration. Results from the dose-escalation stage and the pharmacokinetic stage will be analyzed to develop a treatment dose/schedule for treating 30 additional patients enrolled in the expansion stage.
Each patient enrolled will receive up to three 3-week cycles of STI-6129. After the treatment period, patients will be monitored for up to a year.