In the era of targeted therapy, how to determine the clinical drug dose? ——Take BTKi and PI3Ki as examples

Targeted therapies for PI3K, BTK, and BCL-2 have been approved for chronic lymphocytic leukemia (CLL
).

For example, in dose-exploration studies of various BTK inhibitors, the share of irreversible inhibitors in BTK kinase has been evaluated, but the minimum dose that causes BTK kinase to be completely occupancy has not been determined[1].

Targeted drugs have different dose-response relationships with cytotoxic drugs
.

Therefore, the dose of targeted therapy should be the optimal biological dose (OBD) [2].

Table 1.

The goal of drug development strategies is to achieve maximum biological effects on drug targets, which can be translated into therapeutic effects
.

In clinical trials, the inclusion of these markers can demonstrate: (i) mechanisms, i.

Figure 1 Phase I trial of CLL targeted therapy[1]

In traditional "3+3" clinical phase I trial designs, initially used to study cytotoxic drugs, dose-limiting toxicity (DLT) was used instead of pharmacological markers to guide dose increment
.

It is worth noting that MTD is already established
in the first cycle of treatment.

In 2008, Methodology for the Development of Innovative Cancer Therapies (MDICT) proposed that MTD and pharmacokinetics are reasonable Phase I endpoints
for determining the dose of targeted drugs.

The respective dose selection strategies
in Phase I studies that have now approved CLL therapy are reviewed below.

I.

1、BTK inhibitors

In a Phase I open-label, dose-escalating study of ibrutinib (BTKi) of relapsed/refractory (R/R) B cells non-Hodgkin lymphoma (NHL) and CLL, the level of occupancy of BTK kinase by the covalent inhibitor ibrutinib was used as a biomarker when determining the recommended phase II dose
.

In the phase I trial, patients took ibratinib orally once a day (none of the deaths; OD), 1.

MTD is defined as ≥ 33% of patients experiencing a dose of DLT, or a dose level 3 doses above the lowest dose, provided that BTK kinase is fully occupied and no DLT
is observed.

In this study, MTD of ibrutinib was not achieved, and DLT was reported in only 2 of the 56 patients in the entire cohort
.

Lymphocytosis is a common side effect of ibratinib and other B cell receptor inhibitors, indicating the outflow of lymphocytes from the lymph node chamber
.
Lymphocytosis is transient and not associated
with adverse events, low progression-free survival (PFS), or disease progression.

Figure 2 Structure of approved BTK inhibitors[3]

In the subsequent Phase Ib-II study, ibrutinib treated 85 patients with R/R CLL with 51 patients taking 420 mg OD and 34 patients taking 840 mg OD
.
Corresponding to patients of 70 kg, doses
of 6 mg / kg and 12 mg / kg, respectively.
Full occupancy of BTK was observed at both doses, and the overall response rate was the same
in both groups.
Based on this, a dose of 420 mg is recommended for recurrent CLLs
.
This dose is lower than the recommended dose
for the initial phase I test.

In 2014, the FDA approved ibrutinib for accelerated approval for CLL patients who
had received at least one previous treatment.
In 2016, ibrutinib was approved as a first-line treatment for CLL, compared
to chlorambucil in the RESONATE-2 study.
In 2019, the FDA approved the use of ibuptinib in combination with obinutuzumab (anti-cd20 antibody) for previously untreated adult patients with CLL and expanded the indications in 2020 to include its combination with rituximab (anti-cd20 antibody) for the first-line treatment
of CLL.
It is also approved by
the EMA.
The recommended dose of ibratinib is 420 mg of oral OD
.

Acalabrutinib is the second generation of covalent BTKi
.
Compared with ibuptinib, it is more selective, has fewer off-target effects, and has fewer
adverse events.
In a phase I II study of 61 patients with relapsed CLL, the dose of acalabrutinib in the dose-increasing part of the study was 100-400 mg
.
At the lowest dose it has been observed that BTK kinase is completely occupied (99-100%), so a dose of 100 mg is chosen
.
The half-life of acalabrutinib is 1 h, while the half-life of ibrutinib is 4-13 h
.
Based on these considerations, coupled with the low toxicity of acalabrutinib, it is allowed to be administered twice
a day in phase II studies.

Figure 3 Partially approved targeted therapy for chronic lymphocytic leukemia[1]

Based on research from ELEVATE-TN and ASCEND, the FDA approved acalabrutinib for the treatment
of CLL in 2019.
The drug was approved by the EMA in 2020 as a monotherapy for previously treated CLLs
.
The recommended dose is 100 mg
orally every 12 hours.

Zanubrutinib is another selective covalent BTKi
.
In a phase I study on zanubrutinib for the treatment of R/R B cell malignancies, patients received 40, 80, 160 or 320 mg OD, or 160 mg once every two days; The median BTK kinase occupase occupancy of all doses > 95%, but the 160 mg BID group had a higher persistent BTK occupancy than the 320 mg OD group, which is why
the Phase II dose chose 160 mg BID.
It makes sense to expand the scope of the study by adding a 40 or 80 mg BID regimen because these doses perform as well as the 160 mg OD regimen when given OD
.

Zanubrutinib is an FDA-approved treatment for mantle cell lymphoma (2019) and Waldenström's macroglobulinemia (2021), but has not yet been used in CLL
.
Both 160mg BID and 360mg OD doses have been approved by
the FDA.

Pirtobrutinib is the first generation of non-covalent BTKi in research, which is also effective
against the CLL of BTK C481 mutation.
In a phase I study, pirtobrutinib was used in 323 patients with B-cell malignancies at doses of 25, 50, 100, 150, 200, 250 or 300 mg of OD, and no DLT
was observed.
Based on an estimated target inhibition rate of 96%[4], the recommended dose for Phase II is set to 200 mg/day
.
Target inhibition rates at other doses were not reported
.

2、PI3K inhibitors

Idelalisib is a PI3K inhibitor (PI3Ki) that more specifically blocks p110δ
.
In a phase I study, 54 patients with R/R CLL were treated with idelalisib 300 mg OD or 50, 100, 150, 200, 350 mg BID
.
Since OD administration cannot maintain the same continuous plasma exposure level as BID administration; In addition, patients with BID ≥ 150 mg had longer PFS (32 months and 7 months, respectively)
than those treated at low doses.

Based on this, 150 mg of BID is recommended as a Phase II dose
.
Idelalisib was approved by EMA and FDA for CLL in 2014 at a recommended dose of 150 mg BID
.

Duvelisib is the next generation of dual p110γ/δ PI3Ki
.
In a phase I dose-increasing study, 31 patients with advanced haematological malignancies were treated
with 8, 15, 25, 35, 50, 60, 75, 100 mg duvelisib BID.
The half-life of duvelisib is 5.
2 to 10.
9 h, similar
to ibrutinib.
The long half-life of ibrutinib is the reason why the BID regimen of the drug is not tested, so why not study the OD regimen of duvelisib? Because depending on the occurrence of DLT, MTD is determined to be 75mg BID
.

PI3K signaling inhibition (pAKT) and proliferation inhibition (Ki67) are dose-independent, highest at 25 mg BID
.
The study expanded to 179 patients who received duvelisib BID for 25 or 75 mg
.
≥ level 3 AEs and overall mitigation rates were similar in both cohorts
.
Based on these findings, a lower dose of 25 mg of BID is recommended
.

Figure 4 Structure of a marketed PI3K inhibitor[5]

In 2018, the FDA approved Duvelisib for CLL patients who had received at least two previous treatments
.
The approval is based on the DUO study, in which duvelisib was compared
with anti-cd20 antibody ofatumumab.
It was subsequently approved by
the EMA in 2021.
The recommended dose is 25 mg BID
.

Both Idelalisib and duvelisib have serious toxicity, which leads to the addition of a black box warning
to both drugs.
In addition, the developers of these drugs recently voluntarily revoked the accelerated approval
of follicular lymphoma (FL) due to the inability to complete confirmatory trials.

Umbralisib is p110δ selective PI3Ki
.
In a phase I study of umbralisib in patients with R/R CLL and lymphoma, patients received doses
of 50, 100, 200, 400, 800, 1200, 1800 mg of OD.
Additional cohorts received 200, 400, 800, 1000, 1200, 1800 mg OD (micronylvant form).

The half-life of umbralisib is more than 100 hours
.
Two cases of DLT
were reported in patients who received a micronized formula of 1800 mg/d.
MTD is therefore determined to be 1200 mg/day
.
At doses of 800 or 1200 mg, the plasma concentration of umbralisib is higher than the minimum target exposure of 3000 ng/mL (5.
25 μM).

When plasma concentrations exceed 3000 ng/mL, the tumor load decline tends to plateau
.
Based on this finding, 800 mg/day is recommended as a Phase II dose
.

Umbralisib received rapid FDA approval status in 2020 for the treatment of CLL in combination with anti-cd20 antibody ublituximab, and in 2021 received FDA approval for FL and marginal area lymphoma (MZL) at a recommended dose of 800 mg OD
.

However, an analysis of 6 randomized controlled trials of PI3Ki for the treatment of inert NHL or CLL led to concerns about the lower overall survival rate in the PI3Ki group, and the subsequent voluntary withdrawal of umbralisib from the market for approved indications and the withdrawal of applications for umbralisib in combination with ubluximab for CLL and SLL treatment
.
An FDA review also highlighted the need for caution
in the dosage selection of PI3Ki.

3.
Venetoclax (BCL-2 antagonist)

Venetoclax is a BCL-2 antagonist
.
In a Phase I study of venetoclax, patients first received an experimental dose of 20 mg or 50 mg to detect the development
of tumor lysis syndrome.
The patient then underwent venetoclax after a 3-week incremental program with a final dose of 150, 200, 300, 400, 600, 800, or 1200 mg OD
.
The extended cohort received a final dose
of 400 mg after 5 weeks, starting with 20 mg of OD.
After a dose of 50 mg, venetoclax has a half-life of about 19 h, active
at all studied doses.
For patients treated with < 400, 400, and >400 mg, PFS at 15 months was 58%, 69%, and 77%,
respectively.
The Phase II recommended dose of 400 mg of OD was determined based on remission rates and safety data, but no further details
were provided.

In 2016, the FDA approved venetoclax for use in treated del (17p) CLL patients
.
In a randomized phase III clinical trial comparing venetoclax + rituximab with phendamustine + rituximab in patients treated with R/R CLL in the MURANO study, the FDA approved venetoclax in 2018 for CLL patients who had received at least one previous treatment, with or without the del (17p) mutation
。 The CLL14 study compared the efficacy
of venetoclax + obinutuzumab and chlorambucil + obinutuzumab in patients with CLL who had not been treated before.
After a median follow-up of 28.
1 months, PFS was expected to be 24 months in the venetoclax group at 88.
2% and in the obinutuzumab group at 64.
1%.

Based on this study, the FDA approved venetoclax for all CLL patients
in 2019.
The recommended dose is to increase from 20 mg OD to 400 mg OD
within 5 weeks.

Second, the targeted therapy of lowering the dose does not necessarily affect the efficacy

One of the ethical issues of the "3+3" Phase I trial design (Figure 1) is that some patients may be treated
with suboptimal doses.
However, a systematic analysis of 683 patients treated at doses below, at or above MTD in 24 Phase I trials showed that patients using low-dose targeted therapy did not necessarily show worse outcomes
than other patients in the trial.

This finding also suggests that targeted therapies have different dose-effect relationships
with cytotoxic drugs.
Targeted therapies are less toxic and often reach target saturation before MTD is reached, exerting the greatest tumor suppressive effect
.

One clinical pilot study investigated the pharmacokinetic and pharmacodynamic effects of ibrutinib over three 28-day cycles, from the recommended dose of 420 mg/day by 280 mg/day to 140 mg/day
.
Studies have shown that BTK occupancy, inhibition of downstream signaling and autophosphorylation of BTK (Tyr223), and reduction of plasma chemokines CCL3 and CCL4 levels are similar at three dose levels, suggesting that the currently recommended dose is too high
if the effect of ibratinib is achieved.

To support these findings, several retrospective studies have shown that clinically shown that reducing the dose of ibratinib does not affect the prognosis of CLL
.
In addition, in a prospective CLL study of TP53 abnormalities, ibratinib dose strength did not affect PFS
.
However, although the dosage strength of ibratinib does not affect the prognosis of the patient, forgetting to take ibratinib may be
.

A retrospective study of a phase III clinical trial showed that the median PFS was shorter in patients with continuous ibutinib deletion for ≥ 8 days than in patients with missing <8 days, and a retrospective study of 315 patients in the UK and Ireland showed a 1-year reduction in overall survival (68.
5% vs.
83.
8%)
in patients who had discontinued Ibu for more than 14 days during the first year of treatment compared to the entire cohort.
However, dose interruptions and dose changes do not affect PFS in patients with CLL treated with venetoclax
.

Third, the lessons learned from the toxicity of PI3K inhibitors - intermittent administration

PI3Ki is effective against CLL, but the severe toxicity associated with the first-generation inhibitors idelalisib and duvelisib limits their use
.
Long-term exposure to such targeted therapies has been reported to increase the incidence of adverse events, which makes Phase I studies of traditional 3+3 designs challenging because doses are determined after the
first treatment cycle.
Therefore, alternative experimental designs for these inhibitors should be considered

Nonetheless, the experience we gain from PI3Ki's clinical experience has allowed for the development of more specific next-generation inhibitors and optimized treatment regimens
.
One strategy to overcome therapeutic toxicity is to switch from continuous administration to intermittent administration
.

A retrospective study on idelalisib + rituximab for CLL showed that the treatment benefit far outweighed the duration of treatment (median PFS 29.
6 months and median duration of treatment 11.
9 months).

This finding confirms studies
of time-limited or intermittent administration of idelalisib in prospective clinical trials.
Alternative dosing regimens for both PI3Ki copanlisib and zandelisib have been established and other drugs
are being studied.

Zandelisib is a new generation of p110δ inhibitors whose p110δ occupies longer
than idelalisib.
In a phase I study of zandelisib in healthy volunteers, 60 mg OD was determined to be the recommended phase II dose
based on high inhibition of basophil activation.
A Phase Ib zandelisib study conducted in FL and CLL/SLL showed that the most common AEs were delayed beyond cycle
2.
These AEs can be reversed
by interrupting treatment.

These findings drive Phase I trials with intermittent dosing (7 days/21 days) after two consecutive cycles
.
The basic principle of this time is based on the time
required for regulatory T cells to remultiply.
Preliminary results suggest that intermittent dosing maintains efficacy but reduces the incidence
of delayed grade 3 AEs.
The Phase II TIDAL trial was designed to compare continuous and intermittent dosing regimens in patients with R/R follicular lymphoma, but has been revised to study only intermittent administration (NCT03768505).

The ongoing COASTAL Phase III study only looked at the intermittent treatment regimen
of zandelisib (NCT04745832) in patients with recurrent inert NHL.
In 2020, the FDA approved the Zandelisib Fast Track for the treatment of adult R/R FL patients who
have received at least 2 systemic treatments.

Parsaclisib is another next-generation p110δ inhibitor
.
In a phase I trial in patients with R/R B cell malignancy, intermittent administration was studied
.
Take Parsaclisib 20 mg of OD for the first 9 weeks, followed by 20 mg weekly to reduce delayed AEs
.
This design is based on comparative simulation
of pharmacokinetics and pharmacodynamics of p110α/β/γ/δ inhibitor copanlisib.
No interruption of treatment due to the occurrence of AE in the intermittent administered arm was not reported, while 13% of patients with continuous administered arms discontinued treatment
.
AEs of high-grade were also less
common in the intermittent dosing group.
The Ib/IIa Phase IIa topMIND trial is studying intermediate administration of parsaclisib in combination with tafasitamab (anti-cd19 antibody)
in R/R CLL (NCT04809467).

These studies suggest that intermittent dosing is a strategy
to overcome PI3Ki intolerance.
This strategy is now also being studied in duvelisib
.
In the Phase II TEMPO CLL/SLL trial (NCT03961672), duvelisib is first administered continuously for 3 cycles and then on days 1-2, 8-9, 15-16, 22-23 of each cycle
.

Fourth, with the help of biomarkers to determine clinical dose selection

A review of dose-exploratory studies of CLL targeted therapies suggests that the traditional "3+3" design is still heavily used for new drugs in CLL
.
Because the assay of DLT is less relevant to targeted agents than cytotoxic drugs, additional factors
need to be considered in each trial.

However, only the direct effect of BTKi on drug targets (BTK occupancy) has been consistently studied in multiple trials
.
For PI3Ki, it has proven difficult to determine an end point
.
When a suitable molecular targeting endpoint is available, as is the case with BTKi, using this to guide dose selection may indicate a lower
dose than obtained by the toxicity-driven design.

The finding underscores the fact that in the clinical trials enumerated herein, the minimum BTKi dose that resulted in a full occupancy of BTK was either not determined (the lowest dose tested resulted in a complete occupancy of BTK) or was not reported (only the recommended Phase II dose was reported).

This suggests that there is still room to optimize the dosage of these drugs, which is consistent with many reports that clinically reducing the currently approved dose of ibratinib will not affect CLL outcomes
.

Combination regimens for targeted therapies are becoming increasingly important
in CLL as a strategy to improve remission rates and overcome drug resistance.
This means that patients may experience side effects
of multiple medications.
This should be taken into account when recommending phase II drug doses, as lower drug doses can lead to lower toxicity
.
Drugs used in combination may create synergistic effects, which will expand their respective contributions
.

This further justifies lowering the dose of the drug
.
In vitro treatment of CLL cells with ibrutinib + venetoclax showed that drug synergies occur at doses well below the recommended therapeutic dose
.
The CORAL study (NCT05209308) investigated the combination of zandelisib with venetoclax and rituximab in R/R CLL, including an initial phase I study to reduce the dose of venetoclax, demonstrating that the recommended dose of the target drug requires continuous evaluation
.

In summary, the traditional toxicity-driven "3+3" Phase I trial design remains dominant
in the era of targeted therapy.
Although pharmacodynamic markers have been studied in most clinical trials, these biomarkers have not shown their importance when determining phase II doses
.
If pharmacodynamic markers are used more rigorously to guide dose selection, then in most cases the recommended dose will be lower than the current recommended dose
.

At the same time, reducing the dose can also reduce the treatment-related toxicity and economic stress
experienced by patients and health care systems.
Therefore, the clinical dose of targeted therapeutic drugs should refer more to the optimal biological dose
.
Of course, a suitable pharmacodynamic marker is not easy to determine
.

References:

1， Sigrid S.
Skånland et al; Determining drug dose in the era of targeted therapies: playing it (un)safe? Blood Cancer Journal (2022) 12:123.

2, Excerpted from the "Oncology Specialist Pharmacist Clinical Work Manual".

3，Tiantai Zhang et al; Emerging small-molecule inhibitors of the Bruton’s tyrosine kinase (BTK): Current development, European Journal of Medicinal Chemistry 217 (2021) 113329.

4， Mato AR, Shah NN, Jurczak W, Cheah CY, Pagel JM, Woyach JA, et al.
Pirtobrutinib in relapsed or refractory B-cell malignancies (BRUIN): a phase 1/2 study.
Lancet.
2021; 397:892–901.

5，Minhang Xin et al; Research advances on selective phosphatidylinositol 3 kinase δ (PI3Kδ) inhibitors.
 Bioorganic & Medicinal Chemistry Letters 30 （2020） 127457.

The "Precision Medicine" public account was created
by Dr.
Zhang Zhang, School of Pharmacy, Jinan University.
Dr.
Zhang Zhang is mainly engaged in: 1) the discovery, mechanism of action and druggability of small molecule targeted antitumor drugs; 2) Disease and drug target discovery research based on chemical proteomics
.
As the main finisher, the team developed 4 kinase inhibitors have been transferred to the enterprise for post-development, of which 1 has been listed (orebatinib), 1 newspaper production, 1 in phase I clinical, 1 in the preclinical research stage
.
The laboratory has a sound in vitro and in vitro efficacy, mechanism, druggability, target discovery and confirmation and other technical platforms; It has supported and promoted the research
of the discovery, druggability evaluation, new indications and mechanisms of targeted anti-tumor drugs in many enterprises.