Investigators led by Philip Thompson, MD, a hematologist/oncologist at the center, explained that CLL patients receiving ibrutinib, a Bruton’s kinase inhibitor, “rarely achieve complete remission with undetectable measurable residual disease,” so they stay on the costly treatment indefinitely or until disease progression or accumulating adverse events force a switch to venetoclax.
Using the two agents together, instead of consecutively, may allow strong responders to stop treatment altogether and suboptimal responders to have longer remissions, they said.
“We would not advocate prolonged Bruton’s kinase inhibitor use prior to starting venetoclax in treatment-naive patients, as the safety and efficacy of commencing venetoclax after a 3-month ibrutinib monotherapy phase has been repeatedly demonstrated,” the team said.
However, the investigators noted that their “study was not intended to directly answer the question of whether combination therapy is superior to the current paradigm of sequential monotherapy.” Randomized trials are looking into the matter. The study was published recently as a preprint on ResearchSquare.com and has not been peer reviewed.
Complete remission in over half
The 45 adult subjects had one or more high-risk features for CLL progression and had received at least 1 year of ibrutinib at 140-420 mg once daily, depending on tolerance. They had bone marrow detectable disease at study entry but did not meet criteria for progression. Median duration of ibrutinib at baseline was 32 months, and about half the subjects were on it as their initial therapy.
Venetoclax, a BCL2 inhibitor with a completely different mechanisms of action, was added to ibrutinib for up to 2 years, escalated up to a target dose of 400 mg once daily.
On intention-to-treat analysis, venetoclax add-on improved ibrutinib response to complete remission in 55% of patients; complete remission was defined as less than 1 CLL cell per 10,000 leukocytes in bone marrow on two consecutive occasions 6 months apart.
The rate of undetectable bone marrow disease was 57% after 1 year of combined treatment and 71% after venetoclax completion, at which point 23 patients with undetectable disease stopped ibrutinib along with venetoclax.
Five patients had disease progression at a median of 41 months after venetoclax initiation, one during combined therapy, three during ibrutinib maintenance afterward, and one with Richter transformation after complete remission and discontinuation of all treatment. No patient had died from CLL.
“There has so far been no significant difference noted in” time to residual disease re-emergence, the team said, based on whether or not patients continued ibrutinib after venetoclax add-on.
There was no significant difference in the rate of bone marrow clearance according to the presence or absence of TP53 abnormalities, complex karyotypes, or prior treatment status.
The most common grade 3/4 adverse event was neutropenia in 20% of patients. Nine patients developed nonmelanoma skin cancer during the trial; six were diagnosed with other solid tumors; three came down with grade 3 infections, and two developed myelodysplastic syndrome, both with a prior history of chemotherapy.
No one stopped venetoclax because of toxicity, but about a third of subjects required dose reductions, most often because of neutropenia.
The study was funded by AbbVie, which is commercializing venetoclax along with Genentech. Investigators disclosed ties to both companies and many others. Dr. Thompson disclosed ties to AbbVie, Pharmacyclics, Lilly, Adaptive Biotechnologies, Janssen, Beigene, and Genentech.