PLCG2 L845F

We performed whole exome sequencing on matched tumor and normal samples from 19 CLL patients treated with BTK inhibitors (Ibrutinib, Ibr, 42% (8/19); Acalabrutinib, Acala, 21% (4/19) and Pirtobrutinib, Pirto, 37% (7/19))...The ibr cohort had no prior BTKi therapy whereas 1 patient in the acala cohort had received spebrutinib, another cBTKi...The approach of combining WES data with growth pattern modeling can help unravel the complexities of tumor evolution and drug resistance for the different classes of BTKi. Data on comparative clone growth rates will be presented at the meeting.

We used duplex sequencing, a technology that enables detection of mutations at ultra-low allelic frequencies, to identify mutations in five genes associated with drug resistance in CLL and followed their evolution in two patients who received multiple targeted therapies and ultimately developed disease progression on pirtobrutinib...In patient R001, multiple known resistance mutations in both BTK and PLCG2 appeared following progression on zanubrutinib (BTK p.L528W, p.C481S, PLCG2 S707F, L845F, R665W, and D993H). In contrast, patient R002 developed multiple BTK mutations following acalabrutinib treatment including known resistance mutations p.C481R, p.T474I and p.C481S...For example, BTK p.L528W in patient R001 increased in frequency more than 1000-fold (from CCF 0.02% to 35%), and p.T474I in patient R002 increased in CCF from 0.03% to 4.2% (more than 100-fold). Our data illuminates the evolutionary dynamics of resistant clones over the patients' disease course and under selective pressure from different targeted treatments.

73 patients had only one BTKi (ibrutinib (IBR), 64; acalabrutinib (ACA), 9). 12 pts had multiple BTKis, 8 with two drugs with IBR first followed by ACA (Nf3, 37.5%), vecabrutinib (Nf1, 12.5%), and PIR (Nf4, 50.0%); and 4 with three or more drugs... Our retrospective report summarizes mutations detected during BTKi treatment and shows that BTK L528W can occur during both covalent and non-covalent BTK inhibitor therapy. Four of six patients who progressed on PIR had T474 mutations. In addition, our results may suggest that activating mutations in RAS/RAF/MAPK pathway are related to BTKi resistance.

Novel, non-C481 BTK mutations have been described, including BTK L528W, which has been reported in patients treated with ibrutinib, zanubrutinib, and pirtobrutinib (Maddocks, JAMA Oncol. This is the largest dataset characterizing the incidence and patterns of BTK and PLCG2 mutations in patients with CLL treated with ibrutinib. BTK C481 mutations are the most frequently occurring mutations. PLCG2 mutations occur across the gene at a low incidence per locus (<5%).

There were 29 men, 16 women, median age 65.5 years (range 47-86), 43 patients received ibrutinib and 2 acalabrutinib. Our study shows that BTK and/or PLCG2 mutations were found in 64.4% of patients with progression of CLL during BTKi therapy, and in 35.6% of patients the cause of resistance has not yet been identified. Most mutations in our sample were detected in the C481 codon of BTK gene after 2 years of treatment, suggesting that regular screening with simple PCR tests starting from the second year of treatment is a reasonable approach. NGS may expand data in cases with undetectable mutations.

Here,we generated and comprehensively characterized BTK and PLCG2 mutations conferring resistance to ibrutinib and five different non-covalent BTKi namely pirtobrutinib (LOXO-305), vecabrutinib (SNS-062), nemtabrutinib (ARQ-531), fenebrutinib (GDC-0853), and RN-486. We also found that cells harboring these novel BTK mutations showed differential sensitivity to the covalent vs. non-covalent BTKi. We further demonstrate the potential of venetoclax as follow up treatment upon resistance to non-covalent BTKi.