Clinical Activity of Duvelisib (IPI-145), a Phosphoinositide-3-Kinase-δ,γ Inhibitor, in Patients Previously Treated with Ibrutinib

Introduction: Duvelisib (IPI-145) belongs to an emerging class of therapeutic small molecule kinase inhibitors that target B-cell receptor (BCR) signaling pathways important in various lymphoproliferative disorders. Duvelisib, a novel oral inhibitor of PI3K-δ,γ, has shown clinical activity across a range of hematologic malignancies in an ongoing Phase 1 study.

Some patients (pts) treated with ibrutinib (IBR), a BCR inhibitor that targets Bruton’s tyrosine kinase (BTK), have had disease progression and various mechanisms of IBR resistance have been characterized. Since the mechanism of action of duvelisib differs from IBR, duvelisib was evaluated in a subset of pts previously treated with IBR enrolled in an ongoing Phase I study.

Methods: The study was designed to evaluate the safety, maximum tolerated dose, pharmacokinetics, and activity of orally administered duvelisib twice daily in 28-day cycles. Pts with relapsed/refractory (R/R) hematologic malignancies were enrolled, including pts previously treated with IBR with R/R chronic lymphocytic leukemia (CLL) and aggressive B-cell NHL (aNHL including DLBCL and Richter’s transformation [RT]). Pharmacodynamic studies in CLL pts with measurable disease in the peripheral blood (PB) included flow cytometry to evaluate whether duvelisib inhibits the phosphorylation of AKT (pAKT) at S473 and the CLL cell proliferation index via measurement of Ki67. DNA sequencing on malignant PB cells was performed to determine the mutation status of BTK and other genes involved in hematologic malignancies. Clinical responses were based on IWCLL (2008) criteria and revised IWG (2007) criteria as applicable.

Results: Twelve pts previously treated with IBR were enrolled (R/R CLL, n=6; aNHL, n=6 [DLBCL=2; RT=4]) and received IPI-145 at 25 mg BID (n=2) or 75 mg BID (n=10). The median age in R/R CLL pts was 58 years (y) (42-76), 67% were male, 100% had ≥3 prior systemic therapies, the median time from prior therapy to first dose of duvelisib was 0.34 months (mo) (0.0-1.6), and 67% started duvelisib within 2 wks of last dose of IBR. The median age in R/R aNHL pts was 62 y (36-81), 67% were male, 50% had ≥3 prior systemic therapies, a median time from last prior therapy to first dose of duvelisib of 0.74 months (0.2-3.7), and 50% started duvelisib within 3 weeks of last dose of IBR.

With a median of 4.1 treatment cycles (range 3.0-9.2) in the R/R CLL pts and 2.5 treatment cycles (range 1.8-5.4) in R/R aNHL pts, the safety profile in these pts previously treated with IBR appears consistent with all pts (n=206) with advanced hematologic malignancies receiving duvelisib. The best response observed in R/R CLL pts includes 1 partial response (PR) and 5 stable disease (SD). Of these 6 pts, 2 pts (1 PR, 1 SD) remain on duvelisib for 8 and 9 months, respectively, and 4 pts have discontinued treatment (progressive disease [PD], n=3; physician decision, n=1). The best response observed in R/R aNHL pts included 2 PRs, 1 SD, 2 PD, and 1 not evaluated [NE]. Three pts remain on duvelisib (1 PR, 1 SD, 1 NE) for 3-5 mo and 3 pts have discontinued treatment (progressive disease, n=2; AE, n=1).

The median baseline proliferative index (Ki67) in R/R CLL pts previously treated with IBR (n=5) was substantially higher (22.2% vs 3.8%) than in non-IBR exposed R/R CLL pts (n=23). Across both R/R CLL populations, Ki67 was reduced with duvelisib treatment by Cycle 2 Day 1 (at steady state); however, the effect was more pronounced in non-IBR treated pts. Despite this apparent difference in baseline proliferative index, the IBR previously treated pts with measureable disease in PB (R/R CLL=5), including 3 pts with a known IBR-resistance mutation in BTK (2 with C481S, 1 with C481F) demonstrated reductions of pAKT that were observed within 1 hour of duvelisib treatment and sustained through 24 hours postdose. This pAKT pharmacodynamic response was consistent with results in R/R CLL pts not previously treated with IBR (n=35).

Conclusions: Pharmacodynamic data demonstrates duvelisib inhibits pAKT in R/R CLL pts, including those with an IBR-resistance mutation in BTK, and is consistent with duvelisib having a non-overlapping MOA with IBR. Baseline Ki67 data suggest a more aggressive clinical phenotype in R/R CLL pts who progress on IBR compared to those without previous IBR treatment. Early evidence of clinical activity and a tolerable safety profile suggest additional studies of duvelisib in pts who have progressed on IBR are warranted.