1228 - Single center report on cohort of patients relapsed on Bruton Tyrosine Kinase inhibitors (BTKi)

Introduction
Acquired BTK/PLCG2 mutations remain to be the driver of Bruton Tyrosine Kinase inhibitor(BTKi) resistance. But the underlying mechanisms need to be further explored. In this study, we retrospectively analyzed the clinical and biological characteristics of fifty-nine BTKi-resistant CLL patients in our center and made comparison with responsive cohort.
Method
We retrospectively collected and analyzed clinical and biological characteristics of 229 patients treated with Bruton Tyrosine Kinase inhibitors in our center(between 2014 and 2023), including 59 patients relapsed on BTKi and 170 patients in durable remission. Next-generation sequencing results acquired at the time of disease progression and treatment initiation were used to describe mutational landscape. High sensitivity droplet digital PCR (ddPCR) result was used to validate or detect additional drug resistant hotspot mutation.
Results
The median age was 65(range 33-88) and 55 (range 30-75) years in the durable remission cohort(n=170) and the relapsed cohort(n=59), respectively. Clinical and biological characteristics of above two cohorts at initiation of BTKi treatment were described and compared in Table1. IGHV unmutated status was enriched for the BTKi relapsed cohort. Besides, higher proportion of high-risk biological factors such as del(17p), del(11q) , TP53 aberrations were shown in the BTKi relapsed cohort. BTKi relapsed cohort was also enriched in bulky disease(40.8%) and the medium LDH was 289.3 IU/L (range 125-1191),which was significantly higher than the responsive cohort.
Therefore, the relapsed cohort was enriched for patients with high-risk features, which may explain the inferior treatment response. Consistently, in the relapsed cohort, higher proportion of patients were stratified to high-risk (3-4 point) according to the four-factor prognostic score system developed for predicting the risk of BTKi resistance(50% vs 13.7%, p < 0.0001).
Resistance to BTKi occurred at a median exposure of 29.4 months. Among patients experienced disease progression, 13 were Richter transformation( 12 with diffuse large B-cell lymphoma [DLBCL], 1 with Hodgkin lymphoma [HL] ) while 46 were CLL progression( medium exposure were 9.5 months and 32.6 months, respectively, p < 0.001). The most commonly used fragments in the relapsed cohort were 1-69 (n=11), 4-39 (n=10), 3-48 (n=7), and 3-9 (n=6), which is totally different from the responsive cohort.
Mutational landscape was mapped using NGS results and showed that BTK/PLCG2 mutations were the most common mutation at the timepoint of disease progression. No significant difference was identified except for higher proportion of TP53 mutation(45% vs 24%, p=0.0056) and EGR2 mutation(23% vs 8%, p=0.0105) at the timepoint of disease progression. By integrating NGS and ddPCR results, 54.5%(30/55) patients relapsed on BTKi acquired BTK/PLCG2 mutation. BTK/PLCG2 mutation was detected in 34.3% (12/35), 83.3% (10/12), 70% (7/10) , 50%(1/2)patients relapsed on Ibrutinib, Zanubrutinib, Orelabrutinib and Acalabrutinib, respectively. BTK hotspot mutation analysis was carried and identified that BTKC481 mutation was the most common mutation and was detected in patients treated with all kinds of BTKis. Other mutations at the C481 site include C481R, C481Y, C481F. However, BTKT474 mutation was only detected in patients treated with Orelabrutinib(Figure 1). 4 patients had PLCG2 mutations, including one concurrent with BTK mutation. 6 patients had more than one mutation.
Though higher proportion of TP53 mutation was identified at disease progression on BTKi, most patients had a stable mutation burden(n=20) or even a reduction of mutation burden(n=)8 during the treatment of BTKi according to the matched NGS results of individual patient(Figure 4). 5 patients gained TP53 mutation, all of whom exposed to cytotoxic drugs before BTKi treatment, including 4 exposed to FC-based regimen and 1 exposed to Chlorambucil.
Conclusion
Patients relapsed on BTKi have different clinical and biological characteristics from responsive ones. IGHV unmutated status was dominant in BTKi resistant cohort and the most frequently used fragments were different from that of the responsive cohort. Acquired BTK/PLCG2 mutations remained to be key drivers of BTKis resistance but other high-risk factors may affect the outcome of BTKi treatment and the underlying mechanisms should be explored in order to overcome BTKi resistance.