What happened

Shares of Aptose Biosciences (APTO -4.95%) were sinking 11.9% as of 3:13 p.m. EDT on Monday. The decline came after the company presented preliminary clinical data from an early-stage clinical study evaluating CG-806 in treating relapsed or refractory chronic lymphocytic leukemia (CLL), small lymphocytic leukemia (SLL), or non-Hodgkin lymphoma (NHL). The presentation was included in the American Association for Cancer Research (AACR) Virtual Annual Meeting. 

So what

Aptose's results included only limited information. The company said that the study has completed three dose levels with no drug-related severe adverse events reported so far.

Gloved hand holding a test tube

Image source: Getty Images.

In particular, Aptose mentioned two patients, one treated with a 150-milligram dose of CG-806 twice daily and another treated with a 300-milligram dose twice daily. Neither patient has experienced a dose-limiting toxicity. The dose given to the first patient has been escalated to the 450-milligram level twice per day.

Investors seem to have been disappointed, though, that Aptose didn't reveal much in the way of how effective CG-806 has been in the early-stage study. The company did, however, state that its experimental drug "demonstrates favorable steady state pharmacokinetics," and that phospho-BTK antibody levels were lowered after exposure to plasma from the first patient and were completely abrogated with plasma from the second patient, which hints at the possibility of efficacy.

Now what

Aptose plans to present new data from higher dose cohorts at a future medical conference. The early-stage study evaluating CG-806 is expected to wrap up in late 2020.

The next key catalyst for the biotech stock could take place as early as next week. Aptose is scheduled to provide its first-quarter update on May 5.