Acalabrutinib is a potent and selective BTKi, for which ACP-5862 has been identified as the major pharmacologically active metabolite in plasma. The mean exposure of ACP-5862 is approximately two-fold higher than that of acalabrutinib [13]. ACP-5862 is approximately half as potent as acalabrutinib for BTK inhibition and has a similar kinase selectivity profile [14]. Clinical drug interaction studies with itraconazole and rifampicin have demonstrated that acalabrutinib is a sensitive substrate of CYP3A. Nevertheless, when both the parent drug and active metabolite (total active components) are considered for drug-to-drug interactions (DDI), the magnitude of the CYP3A DDI seems to be much less significant [15]. Differently from ibrutinib, results from a physiologically based pharmacokinetic model have demonstrated that no dose adjustment is needed for concomitant administration of acalabrutinib with moderate CYP3A inhibitors [16]. In a population pharmacokinetic analysis on acalabrutinib and ACP-5862, drug solubility was shown to decrease with increasing pH [17]; as a consequence, its absorption greatly depends on gastric pH changes. For the same reason, the use of proton pump inhibitors, associated with a reduction in area under the curve, should be avoided in patients on acalabrutinib [18].

In competitive binding assays using a panel of 395 non-mutant kinases, acalabrutinib compared to ibrutinib has shown a greater selectivity towards members of the TEC family of kinases [19]. In particular, some of the ibrutinib off-target kinases such as EGFR, ITK and ERB-B2 receptor tyrosine kinase were not inhibited by acalabrutinib [20,21]. Additional off-target kinases, which also include tyrosine protein kinase, were inhibited by acalabrutinib in vitro at high nanomolar concentrations [21,21].

The activity and safety of single-agent acalabrutinib were first evaluated in 134 R/R patients of a single-arm phase 1/2 trial (ACE-CL-001) [22]. Acalabrutinib was administered orally at different schedules once or twice daily until progression or intolerance. Median of prior lines was 2; unmutated IgHV mutational status (uIGHV) was reported in 75% of patients while 31% carried 17p deletion (17p−). After a median follow-up of 41 months, most patients (56%) remained on treatment; primary reasons for discontinuation were progressive disease (21%) and adverse events (AEs, 11%). Median progression-free survival (PFS) was not reached and estimated 45 months PFS was 62%. Importantly, because of improved trough BTK occupancy with twice-daily dosing, 100 mg bid was determined as the preferred dose for further trials.

Of note, a cohort of 99 treatment naïve (TN) patients not eligible to chemoimmunotherapy (CIT) was also evaluated in the ACE-CL-001 [23]. After a median follow-up of 53 months, median PFS was not reached and the estimated 4-year PFS rate was 96% (82% in patients with del(17p) and/or mutated TP53, and 91% in patients with complex karyotype).

As acalabrutinib does not inhibit ITK, its reduced interference with antibody-dependent cellular phagocytosis (ADCP) and antibody-dependent cellular cytotoxicity (ADCC) may result in a potential effective combination with anti-CD20 antibodies [24]. In another phase I/II acalabrutinib was administered in combination with obinutuzumab (O-Acala) in 45 patients (19 TN; 26 relapsed/refractory, R/R) [25]. Median observation was 39 and 42 months in TN and R/R, respectively. Overall response rate was 95% in TN and 92% in R/R with up to 32% of the previously untreated patients achieving a complete remission (CR) (8% in previously treated). Median PFS was not reached in both cohorts. This regimen appeared tolerable, with a rate of discontinuation due to AEs (11%) consistent with that of the monotherapy schedule with a similar time of follow-up [22,25].

On this background, two phase III studies demonstrated the superiority of acalabrutinib +/− obinutuzumab over CIT both in relapsed/refractory and previously untreated CLL.

The multicenter randomized phase III ASCEND trial enrolled 310 patients with ≥1 previous line who received acalabrutinib monotherapy (n = 155) or investigator’s choice (idelalisib plus rituximab, n = 119, or bendamustine plus rituximab, n = 36) [26]. Patients had a median of 2 prior lines (range, 1–10), but prior Bcl-2 or BTK/PI3K inhibitors were not allowed. With up to 4 years follow-up, acalabrutinib significantly prolonged investigator-assessed PFS versus the investigators’ choice arm (42 months PFS 62% versus 19%) with a trend for overall survival (OS) advantage, although more than half of participants enrolled in the comparator arm crossed over to the BTKi. The acalabrutinib PFS benefit was confirmed across all prognostic subgroups. Adverse events led to drug discontinuation in 23% of patients.

In the setting of previously untreated patients, the phase III ELEVATE TN trial randomized elderly/unfit patients (fitness defined by age, CIRS, creatinine clearance) to receive either acalabrutinib alone (n = 179), together with obinutuzumab (O-Acala, n = 179) or obinutuzumab + chlorambucil (O-Chl, n = 177) [27,28]. Regarding biological characteristics, 9% of the patients had del(17p), 18% del(11q), 63% unmutated IGHV and 17% had a complex karyotype. Apart from the expected clear advantage of the two acalabrutinib- arms over O-Chl, interestingly at the 5-year update a higher rate of progression/deaths was observed in the monotherapy versus O-Acala arm (5 years PFS 72% vs 84%, respectively, p = 0.0259) with also a trend toward superior OS with the combination. Of note, such PFS difference did not emerge in 17p−/TP53 mutated patients.

All these studies have shown a favorable toxicity profile of acalabrutinib and demonstrated its superiority over both CIT and another BCR inhibitor, idelalisib. Nevertheless, only one trial so far has provided a head-to-head comparison of acalabrutinib with ibrutinib. The phase III ELEVATE RR, in fact, was designed as a non-inferiority study, directly comparing the two BTKis in high risk CLL with 17p and/or 11q deletion [29]. Both agents were given as monotherapy. In this difficult-to-treat population, no differences were reported between the two arms neither for overall response rate (ORR) (acalabrutinib versus ibrutinib 81% versus 78%) nor for PFS (38.4 months in both groups), thus meeting the primary endpoint of the study.

Differently from the previously treated population, no studies have so far directly compared acalabrutinib to ibrutinib in the setting of treatment naïve. However, at indirect comparison, acalabrutinib given both as monotherapy or combined with obinutuzumab, appears to be not inferior to ibrutinib + obinutuzumab in terms of PFS  [30,31,32,33].

Zanubrutinib (BGB-3111) is another next-generation BTK inhibitor that has already shown high specificity and oral bioavailability in preclinical studies, with reduced off-target activity in vitro enzymatic and cell-based assay [34]. The drug is rapidly absorbed, with Cmax observed in about 2 h after its oral administration. In the phase I study, the Cmax and the area under the concentration–time curve increased proportionally with the increase of zanubrutinib dosage. Importantly, among the approved BTKi, zanubrutinib seems to be less susceptible to PK modulation by intrinsic and extrinsic factors, thus resulting in consistent, sustained therapeutic exposures [35].

In the phase I, first-in-human AU-003 trial, zanubrutinib was offered as a single agent in 144 CLL/SLL at 40, 80, 160, or 320 mg once daily or 160 mg twice daily [36]. Although median BTK occupancy was comparably high with all dosages in peripheral blood, in lymph node tissues this was more frequently >95% with 160 mg bid than 320 mg once daily, thus leading to the zanubrutinib recommended phase II dose of 160 mg bid.

It is important to highlight that at this dosage, when adjusted for plasma protein binding, zanubrutinib exposure is approximately eight-fold higher than that observed with ibrutinib 560 mg daily [37,38]. Nevertheless, no clear differences in safety or activity were reported between the single or the spliced daily dose.

With a median follow-up of 47.2 months, 123 patients (22 TN and 101 R/R) were evaluated for efficacy. Median prior lines in the previously treated cohort was 2 (range 1–10). About one third of cases in both cohorts had TP53 abnormalities. Median treatment duration was 43 months. ORR was 100% and 95% in TN and R/R patients, respectively. Responses deepened over time reaching about 20% of complete remissions in both cohorts at the 4-year cut-off. Median PFS was not reached in TN and estimated at 61.4 months in R/R. Overall, 46 patients (37.4%) discontinued treatment, mostly due to progressive disease (21% of the whole cohort). Importantly, the occurrence of AEs was the main reason for zanubrutinib definitive interruption only in 9.8% of patients in this series [36].

Comparative phase III studies with zanubrutinib include the SEQUOIA trial [39], conducted in a previously untreated population, and the ALPINE study on relapsed or refractory patients [40]. The SEQUOIA was aimed to compare zanubrutinib (group A) with bendamustine+rituximab (BR, group B) in 479 older or comorbid patients, with PFS as the primary endpoint. Given the well-known lack of efficacy of BR in 17p deleted CLL, 111 patients carrying such aberration were directly assigned to the arm C of the protocol and received zanubruitnib open (group C) [39,41]. With a median follow-up of approximately 2 years, the primary endpoint was met for the cohort comparing zanubrutinib with CIT (hazard ratio 0·42 [95% CI 0·28–0·63]; p < 0·0001). Considering all the limitations of indirect comparisons, 86% 2-years PFS with zanubrutinib appears to be consistent with that of ibrutinib (85%) and acalabruitnib (87%) at similar timepoints, and comparable to that of high-risk patients with 17p deletion enrolled in group C (18 months-PFS 88.6%). The PFS results of group C are instead difficult to compare with those of other BTKi due to the small number of patients with TP53 aberrations enrolled in similar first-line trials [5,42], but appear to be comparable to the rate reported at 2 years by Ahn et al. with ibrutinib [43]. The PFS advantage over CIT was present in all risk-subgroups with the exception of IGHV mutated patients, which are those known to benefit most from CIT. As expected from the relatively short follow-up, the possibility to cross over and the presence of an intensive regimen as comparator, no differences in overall survival between zanubrutinib and BR emerged at the last update.

Zanubrutinib met and outperformed the primary non-inferiority endpoint, achieving significantly superior ORR (ORR 78.3% versus 62.5% with zanubrutinib versus ibrutinib, respectively). This difference showed a greater magnitude in 17p deleted patients (ORR 83.3% and 53.8%, respectively). Importantly, ORR in this trial was defined as the rate of partial + complete remissions (PR, CR), and excluded patients achieving PRs with lymphocytosis (PR + Ly). Lymphocytosis is an expected pharmacodynamic event with BTKi, which was shown to be not associated with inferior efficacy or long-term survival outcomes with ibrutinib [38]. Taking this into account, the clinical significance of achieving a deeper response (that excludes the persistence of lymphocytosis) needs be clarified over time.

With a median follow-up of 29.6 months, zanubrutinib showed a significantly longer PFS in respect to ibrutinib (median PFS not reached for zanubrutinib, 34.2 months for ibrutinib, HR: 0.65 [95% CI, 0.49–0.86]; 2-sided p = 0.002) both as per investigators and independent reviewer committee assessment. The PFS results favored zanubrutinib consistently across major pre-defined subgroups including IGHV status and patients with del(17p)/TP53. Median OS was not reached in both groups with no statistic differences observed (HR due to death 0.76; 95% CI, 0.51 to 1.11). Finally, compared to ibrutinib, zanubrutinib showed a higher ORR (83.5% versus 74.2%) as per investigators’ assessment, which was consistent with that of the independent review committee.

With 8 years of follow-up data since its initial pivotal study [38], the increasing knowledge on the specific toxicity profile of ibrutinib has improved the early recognition and management of specific AEs, such as supraventricular arrhythmias, hypertension and increased risk of bleeding. Notably, although the incidence of grade ≥3 AEs from ibrutinib initiation tends to decrease while on treatment, the prevalence of some event of special interest remains stable over time, leading to clinically significant rates of discontinuations or dose reduction seen in landmark clinical studies of the BTKi [5,6,7,8,9].

It is important to underline that in several phase III trials ibrutinib was administered in different categories of patients stratified according to age and comorbidities and, even with long follow-up, showed a similar rate of toxicity-related discontinuations (21–24%) across 3 to 8 years of observation [5,6,7,9].

However, when focusing on specific adverse events, some difference in toxicity emerged in the elderly. At indirect comparison, in fact, patients enrolled in the ibrutinib + rituximab arm of the ALLIACE 041202 trial (median age, 71 years) [5] showed significantly higher rates of grade 3–5 infections, atrial fibrillation, bleeding and hypertension than young patients receiving the same combination in the ECOG1912 study (median age 58 years) [9].

Next-generation BTKi were developed to maximize the on-target inhibition of BTK with a supposed reduction of adverse events in vivo.

Direct comparisons between BTKi in first-line studies are missing. Long-term data from ELEVATE TN suggest that the addition of obinutuzumab to acalabrutinib does not increase toxicity [28].

Despite all the limitations of indirect comparison, when considering two first-line studies analyzed at a similar timepoint, the ILLUMINATE (experimental arm: ibrutinib + obinutuzumab) and the ELEVATE TN trials [6,28], at 45- and 46.9-months follow-up, respectively, the safety profile appears to favor acalabrutinib-based treatments over ibrutinib. In particular, AF, reported in 3.9 and 6% with acalabrutinib arms, was recorded in 15% of patients randomized to ibrutinib + obinutuzumab in the ILLUMINATE study. More important, the discontinuation rate for toxicity in patients treated with acalabrutinib in the ELEVATE TN is almost half that of patients receiving ibrutinib + obinutuzumab (12.3–12.8% versus 22%) or even of those treated with single-agent ibrutinib in the 5-year follow-up of the RESONATE-2 trial (16%) [45].

Differently from the first-line treatments, reliable data on the direct comparison of ibrutinib versus acalabrutinib are provided by the ELEVATE RR trial in previously treated patients.

According to study results, overall fewer patients receiving acalabrutinib versus ibrutinib developed severe adverse events, thus translating to a lower rate of definitive discontinuations due to toxicity (14.7% versus 21.3%, respectively) [29]. The incidence of AF and severe infections was a specific secondary endpoint of the study. While no differences were observed for the infection rate, atrial fibrillation, as well as hypertension, were reported in a significantly lower proportion of patients receiving acalabrutinib versus ibrutinib. Moreover, acalabrutinib was associated with a lower incidence and exposure-adjusted incidence of AF, hypertension and bleeding, which was significant in patients with no prior history of such events.

Again, direct comparison is available in relapsed/refractory population. Treatment management favored zanubrutinib in respect to ibrutinib in the ALPINE trial [40], with a lower number of patients discontinuing therapy in the zanubrutinib arm. Interestingly, atrial fibrillation was the only event of special interest significantly more frequent in patients randomized to ibrutinib versus zanubrutinib (13.3% vs 5.2%), while incidence of hypertension and bleeding were superimposable between the two arms. Similarly, no differences were reported in the rate of severe, serious and fatal adverse. As previously seen in other studies, neutropenia was slightly higher with zanubrutinib, not translating to increased rate of infections.

Of note, six grade 5 cardiac events occurred during the study, all in the ibrutinib arm.

Clinical trials on the use of an alternative BTKi in case of intolerance are available for both acalabrutinib and zanubrutinib [44,46]. Intolerance was defined as the persistence or recurrence of adverse events considered to be related to the first BTKi. Both these phase II studies support the use of an alternative BTKi in case of toxicity, as most patients were able to continue to be treated with an irreversible BTK or presented a recurrence of the adverse event at a lower intensity.

To summarize, any conclusion about the potential advantage of next-generation BTKis in first line should be taken with caution as it comes from cross-trial comparisons and might fail to recognize inherent differences across studies. On the other hand, the availability of two randomized, controlled phase III trials allows a more appropriate comparison of the safety profile of ibrutinib versus next-generation inhibitors in a previously treated population. Compared to ibrutinib, incidence of atrial fibrillation is lower with both acalabrutinib and zanubrutinib and this is probably the event affecting most ibrutinib management. As regards this specific toxicity, zanubrutinib seems to be even more safe than acalabrutinib at indirect comparison. Nevertheless, it should be kept in mind that no definitive conclusions can be drawn so far, given the shorter follow-up (~2–4 years) of zanubrutinib. New onset or worsening of hypertension does not differ between ibrutinib and zanubrutinib, but seems to be significantly reduced in patients receiving acalabrutinib, thus suggesting favoring the use of this inhibitor in this particular setting.

Several other next-generation covalent BTKi are currently under investigation for B-cell malignancies including CLL.

Spebrutinib (CC-292) inhibits BTK by binding irreversibly the same cysteine 481 of ibrutinib [47]. In a phase 1 study on 84 pretreated CLL/SLL patients, including 23.8% with 17p deletion and 21% with 11q deletion, spebrutinib given at different dosages was overall well tolerated and led to a median response duration up to 11 months in the 1000 mg cohort [48]. Although effective even in high-risk cytogenetics, clinical results with spebrutinib are inferior to those already consolidated with ibrutinib, thus resulting in the interruption of its clinical development.

Orelabrutinib (ICP-0229) in vitro showed to be more selective than ibrutinib at 1 µM concentration while targeting BTK with >90% inhibition. The BTKi is currently approved in China for previously treated CLL/SLL [49], following the results of a phase II trial on 80 previously treated patients. In this population orelabrutinib allowed for the achievement of an ORR of 93.8% with an estimated 30-months DOR of 70.6% that was maintained regardless of biologic risk. Of note, no cases of AF or severe hypertension were reported while only one patient developed a treatment-related major bleeding [50].

Tirabrutinib (ONO/GS-4059) as well demonstrated greater selectivity on BTK than ibrutinib. Its activity on B-cell malignancies was reported in 90 R/R patients including 25 CLL. All but one obtained a response within 3 months from treatment start. Low grade diarrhea was the most common AE reported occurring in 18% of patients; hematologic toxicity was the most common grade ≥3 event recorded in CLL, while no cardiac toxicities occurred [51]. Tirabrutinib-based combinations were also analyzed; however, a significant increase of therapy-related toxicities was noted with association to other agents with a reported rate of grade 3 or higher adverse events >70% [52,53].