Allogeneic Stem Cell Transplantation (alloHSCT) for Chronic Lymphocytic Leukemia (CLL) in the Era of Novel Agents

Introduction: Prior to effective novel agent (NA) approval for CLL, alloHSCT was recommended for CLL patients (pts) with early relapse/refractory disease after purine-analogs or deletion 17p (del17p) or TP53 mutation (TP53mut) based on expert consensus and retrospectively demonstrated overall survival (OS) advantage. Since 2014, approvals of ibrutinib (ibr), venetoclax (ven), and PI3K inhibitors (PI3Ki) have led to fewer alloHSCT for CLL. While NAs are indisputably effective, many pts will eventually progress through all available NAs. In the absence of data-driven consensus regarding role of alloHSCT for CLL, decision about proceeding to transplant is currently based on disease and transplant risk, response to NAs, and pt preference. This study of CLL pts who underwent alloHSCT following NA therapy (tx) aimed to help define the role of this potentially curative modality in the era of NAs. Methods: This multicenter, retrospective cohort study examined CLL pts who underwent alloHSCT following treatment with ≥ 1 NA, including baseline clinical, prognostic, and transplant characteristics, tx preceding alloHSCT, transplant outcomes, and tx following alloHSCT. Complex karyotype (CK) and CLL status [complete remission (CR), partial remission (PR), stable disease (SD), and progression of disease (POD)] were defined per iwCLL criteria (Hallek, et al. Blood 2018). Univariate analyses utilizing COX regression evaluated association between pre-alloHSCT factors and progression free survival (PFS). PFS, OS, and non-relapse mortality (NRM) were estimated using Kaplan Meier and life table methods. Other statistics were descriptive. Results: 69 pts with CLL underwent alloHSCT following ≥ 1 NA across 14 US and EU centers, including 6 pts with Richter's transformation (RT) prior to alloHSCT. Table 1 describes baseline characteristics. Prior to alloHSCT, 78% received ibr (n=53), 39% ven (n=25), 20% PI3Ki (n=13), and 36% ≥ 2 NAs. 90% (n=62) received a NA immediately preceding alloHSCT [n=32 ibr (16% CR, 75% PR, 9% SD/POD), n=25 ven (52% CR, 40% PR, 8% SD/POD), 4 PI3Ki (75% PR, 25% SD/POD), 1 IMiD]. With a median (med) follow up 28 months (mo; range 1.2 -85), med PFS and OS from alloHSCT for the entire cohort were not reached (Figure 1A, B). PFS and OS for pts with CLL (excluding RT pts) from alloHSCT were 60% and 82% at 24 mo respectively. Poor risk disease characteristics (TP53mut, del17p, CK), prior NA exposure (ibr, ven, PI3Ki, ≥2 NAs), and transplant characteristics (matc