An Open-Label, Phase I Study of the Polo-like Kinase-1 Inhibitor, BI 2536, in Patients with Advanced Solid Tumors

As our understanding of cancer biology has increased considerably in recent years, so too have the options for potential novel targeted treatments, including those targeting the cell cycle.

Mitotic kinases of the polo family, which are highly conserved in all eukaryotes, have been identified as important regulators of cell division and its checkpoints (1, 2). In mammals, four members of the family have been identified: polo-like kinase (Plk)–1, Plk-2, Plk-3, and Plk-4 (1, 3, 4). Each member of the Plk family contains a serine/threonine kinase domain and a polo-box domain, which serve as potential target sites for drug development (3, 4).

Plk-1, the most extensively characterized mammalian Plk, controls several key steps in the passage of cells through mitosis, including entry into mitosis, centrosome separation and maturation, metaphase to anaphase transition, and mitotic exit, and the onset of cytokinesis (3, 4). Importantly, Plk-1 seems to be specifically active during mitosis and does not seem to have activity in nondividing cells, making it an attractive and selective target for drug development (5). Of relevance is the observation that Plk-1 is abundant in various human cancers with high mitotic activity, such as non–small cell lung cancer (6).

Plk-1 inhibition or depletion in vitro results in DNA damage and a subsequent blockade of cancer cell proliferation and induced apoptosis (7). BI 2536 is a highly selective and potent small-molecule Plk-1 inhibitor (inhibitory concentration₅₀ = 0.83 nmol/L), showing >1,000-fold selectivity versus a large panel of other kinases (8). Preclinical studies in human cancer cell lines have shown that BI 2536 disrupts spindle assembly, resulting in mitotic arrest and inducing apoptosis (8, 9).

This phase I dose-escalation study is one of two first-in-man trials undertaken with BI 2536 in cancer patients. Both studies began simultaneously; one was designed to investigate single dosing of BI 2536 and showed that BI 2536, administered as an infusion once every 3 weeks, has a favorable toxicity profile (10). Here, we report the findings of the second study, which was designed to investigate repeated dosing of BI 2536 and assessed two different dosing schedules in cancer patients with advanced or metastatic solid tumors.

The primary objective of this study was to determine the maximum tolerated dose (MTD) of BI 2536 in each of two treatment schedules administered in patients with advanced solid tumors. Secondary objectives included evaluation of the safety, efficacy, and pharmacokinetics of BI 2536.

This phase I, open-label, dose-escalation study was conducted at two clinical sites in Germany between August 2004 and December 2007. A standard 3 + 3 design was used. The MTD was defined as the highest dose at which not more than one of six patients experienced dose-limiting toxicity (DLT) in the first treatment course. Once the MTD was determined, enrollment into higher dose cohorts was suspended, and MTD patient cohort was expanded. A dose-escalation accelerated titration design was used, with 100% dose increments until the first reports of grade 2 or higher drug-related toxicity. Escalation steps of no more than 50% were permitted thereafter. If a DLT was observed in one of six patients in a treatment cohort, increments were to be no more than 35%.

This study comprised two parts and investigated two different BI 2536 dosing schedules. Patients investigated during the first part of the study initially received a single i.v. dose of BI 2536 25 mg over a 1-hour infusion period on days 1 and 8 of a 21-day treatment course. Once the MTD for this treatment schedule had been established, a second treatment schedule was investigated because the agent had shown a favorable safety profile and administration schedules for phase II could not be selected based on preclinical data. The 24-hour infusion was chosen based on the hypotheses that prolonged mitotic arrest by means of Plk-1 inhibition may maximize antitumor efficacy and that lower peak plasma levels may give rise to fewer side effects. A protocol amendment was made, introducing a second single-dose treatment schedule, in which BI 2536 was administered as a 24-hour infusion on day 1 of a 3-week treatment course, and the starting dose was increased to 50 mg; the MTD was determined as described above. This dosing schedule aimed to optimize the administration schedules for any future phase II studies.

Patients were assigned to escalating dose cohorts in order of their admission into the study. Patients who reached day 21 without experiencing disease progression or excessive toxicity were eligible to receive additional courses of BI 2536 treatment. Patients who had experienced a DLT could also be considered eligible for a further treatment course at the next lower dose if they experienced benefit from therapy and had recovered from adverse events.

Adult patients who had failed conventional treatment but had a life expectancy of at least 6 months and an Eastern Cooperative Oncology Group performance score of 0 to 2 were included in the study. Patients were required to have confirmed diagnosis of advanced, nonresectable, and/or metastatic solid tumors and evaluable tumor deposits.

Patients with known brain metastases, active infectious disease, or a serious illness thought to interfere with the protocol were excluded from the study, as were those who were treated with other investigational drugs or had participated in another clinical trial within the 4 weeks before the start of therapy or concomitantly with this trial. Patients with a known secondary malignancy that required therapy, abnormal hematologic values, or impaired renal or liver function were also excluded.

The trial was carried out in compliance with the protocol and the principles laid down in the Declaration of Helsinki (1996 version), in accordance with the International Conference on Harmonization Harmonised Tripartite Guideline for Good Clinical Practice, and in accordance with applicable regulatory requirements. Written informed consent was obtained from each patient before their participation in the study.

Concomitant medications were given as clinically necessary to provide adequate care and were recorded in the Case Report Form, except for vitamins or nutrient supplements. The symptomatic treatment of adverse events or tumor-associated symptoms was allowed. Additional chemotherapy, immunotherapy, hormone therapy, or radiotherapy were not permitted during the study. Administration of prophylactic growth factor support was discouraged during treatment cycle 1 so that assessment of the rate and duration of neutropenia would not be compromised during determination of DLTs; however, therapeutic administration of growth factor support as required was allowed per protocol. Patients who received growth factor support during the first treatment cycle were excluded from analyses for the primary endpoint of this study, unless a DLT had occurred already before initiation of growth factor support.

Objective response was assessed by tumor measurements and evaluated according to Response Evaluation Criteria in Solid Tumours (RECIST; ref. 11). Target lesions were assessed and defined as a complete response (CR), partial response (PR), stable disease, or progressive disease. Patients were assessed at screening and then at the end of every other treatment course; however, tumor measurements at earlier time points after the start of study treatment could be done if clinically indicated based on the judgment of the investigator. Secondary efficacy assessments included progression-free survival and overall survival. For the progression-free survival analysis, patients who had not experienced progression or death were censored at the date of the last contact between the patient and the investigator. For the overall survival analysis, patients who had not died were censored at the date of the last physical contact between the patient and the investigator or (if later) the latest end date of an adverse event.

Adverse events according to Common Terminology Criteria for Adverse Events (CTCAE) version 3.0, laboratory evaluations, electrocardiogram, performance status, physical examination, and vital signs were all used to determine safety. Adverse events with an onset within 21 days after the last administration of BI 2536 were considered as occurring following treatment. DLT was defined as a drug-related CTCAE grade 3 or 4 nonhematologic toxicity (except reversible emesis or diarrhea) or drug-related CTCAE grade 4 neutropenia for ≥7 days and/or complicated by infection or grade 4 hematologic toxicity other than neutropenia. After 14 patients had been included in the study, a protocol amendment was made for patients assigned to the day 1 and 8 dosing schedule; if the day 8 administration was omitted because of treatment-related grade 2 nonhematologic or grade 3 hematologic toxicity, this was also considered to be a DLT. The MTD was defined on the basis of DLT observed during the first treatment course (3 wk).

Blood samples for the evaluation of pharmacokinetic variables were collected on day 1 and 8 at predose, 20, and 40 minutes, and 1, 1.5, 2, 4, 6, 10, 24, and 72 (day 1) or 96 (day 8) hours following drug administration for both the day 1 and 8 dosing schedules. For the day 1/24-hour dosing schedule, samples were collected predose and at 30 minutes, 1, 4, 8, 20, 24, 24.5, 25, 27, 29, 48 (optional), 72, and 168 hours following drug administration. BI 2536 plasma concentrations were determined by high-performance liquid chromatography coupled to tandem mass spectrometry at the Department of Drug Metabolism and Pharmacokinetics, Boehringer Ingelheim Pharma GmbH and Co. KG, Biberach, Germany.

Safety, efficacy, and pharmacokinetic characteristics were analyzed in an exploratory and descriptive manner. Noncompartmental pharmacokinetic parameters were determined using WinNonlin or another validated program. All patients who received at least one dose of BI 2536 (treated set) were included in the efficacy and safety analyses. The escalation scheme in this trial was such that there was at least an 80% chance that at least two patients would experience a DLT for a given dose if the chance of a DLT was ≥42% for each patient.

Patient demographics and clinical characteristics are summarized in Table 1. A total of 70 patients met the study entry criteria and were enrolled, 44 of whom received treatment according to the day 1 and 8 dosing schedule. Thirty-seven of these completed at least one treatment course. Twenty-six patients were enrolled and treated on the day 1/24-hour dosing schedule. Twenty-four patients completed at least one treatment course; the other two patients (both receiving BI 2536 150 mg in the 24-h continuous regimen) died before the end of the 21-day observation period because of events unrelated to study drug. The disposition of patients for both dosing schedules is presented in Table 2.

For the day 1 and 8 dosing schedule, the median number of completed courses was 1; a maximum of eight courses were completed. For the day 1/24-hour dosing schedule, the median number of courses completed was 2; a maximum of seven courses were completed.

DLTs are shown in Table 3. In the day 1 and 8 dosing schedules, the MTD was determined to be BI 2536 100 mg per administration, that is, a total dose of 200 mg per course. The first two patients treated at the dose of BI 2536 200 mg experienced treatment-related DLTs during the first treatment course. One patient experienced grade 4 neutropenia and grade 4 thrombocytopenia. The second patient experienced grade 3 neutropenia on day 8, necessitating omission of day 8 BI 2536 administration. Three additional patients were treated with BI 2536 150 mg. Two DLTs within this dose cohort (both grade 3 neutropenia) resulted in three additional patients being treated with BI 2536 100 mg. Because only one of six patients treated in this dose cohort experienced a DLT (grade 4 neutropenia), an intermediate dosing level of BI 2536 125 mg was introduced. Two of the five patients treated with BI 2536 125 mg experienced a DLT (both grade 4 neutropenia). The MTD for the days 1 and 8 dosing schedule was therefore established as 100 mg of BI 2536 per administration or 200 mg per course.

With regard to the day 1/24-hour dosing schedule, although the two fatalities in the BI 2536 150-mg cohort were not deemed to be drug-related by the investigators, further patients were recruited at this dose level, and any planned dose escalations were reduced as a precaution. In this regimen, reported DLTs during the first treatment course were grade 4 neutropenia (one patient each receiving BI 2536 150 and 175 mg), grade 4 thrombocytopenia (one patient receiving BI 2536 225 mg), and grade 3 exanthema (one patient receiving BI 2536 175 mg). After recruitment of six patients into the BI 2536 225-mg cohort, it was decided that the day 1/24-hour dosing schedule would not be extended into phase II trials and the dose would not be further escalated, and patient accrual was halted. BI 2536 225 mg was shown to be tolerable, therefore the same total dose given in other treatment schedules could also be given in the infusional regimen; however, no formal MTD was defined for this dosing schedule, and no treatment cohort expanded.

During the study, all patients experienced at least one adverse event. Of the 70 treated patients, 41 (59%) experienced a drug-related adverse event; these included 23 patients (53%) in the day 1 and 8 dosing schedules and 18 patients (69%) in the day 1/24-hour dosing schedule. The most frequently reported adverse events, regardless of relationship to treatment, were gastrointestinal events (46 patients; 66%), general disorders and administration site conditions (45 patients; 64%), and blood and lymphatic disorders (43 patients; 61%). Treatment-related adverse events reported during the study are summarized in Table 4. Most frequently reported treatment-related adverse events were neutropenia (32 patients; 46%), leucopenia (28 patients; 40%), and anemia (10 patients; 14%).

With the exception of neutropenia and leukopenia, grade 4 drug-related adverse events were rare (Table 5). Three patients experienced grade 3 drug-related nonhematologic events; one patient treated with BI 2536 25 mg in the day 1 and 8 dosing schedule experienced a grade 3 hemorrhage, while two patients treated with BI 2536 175 mg in the day 1/24-hour dosing schedule experienced grade 3 events (infection and rash). No patients experienced grade 4 nonhematologic events related to the study treatment.

Clinical laboratory evaluations showed that hematologic adverse events were frequently reported in patients but were generally mild-to-moderate. Anemia was the most frequent adverse event, with all but one patient reporting less than normal levels of hemoglobin at some point during the study. Thrombocytopenia was reported in 28 patients (40%), with six patients experiencing grade 2 to 3 events as their worst overall grade. Three patients experienced grade 4 thrombocytopenia. Neutropenia constituted the main side effect reported in this study. Decreased neutrophil counts were reported in 41 patients (59%) in total [25 patients (57%) from the day 1 and 8 dosing schedule and 16 patients (62%) from the day 1/24-h dosing schedule]. Severe neutrophil abnormalities (grade 3-4) were reported in 16 patients (36%) in the day 1 and 8 dosing schedule and in 13 patients (50%) in the day 1/24-hour dosing schedule. A total of 15 patients (21%) experienced grade 4 neutropenia.

Abnormal liver enzyme activity [alanine transaminase (ALT) and aspartate aminotransferase (AST)] was the most commonly reported biochemical abnormality, but events were mostly mild. In the day 1 and 8 dosing schedules, 15 patients (34%) had abnormal ALT values and 25 (57%) had abnormal AST values, whereas in the day 1/24-hour dosing schedule, nine patients (35%) had abnormal ALT and 17 patients (65%) experienced abnormal AST levels. Across all patients, one patient experienced a grade 3 ALT abnormality (day 1 and 8 dose schedules; 50-mg cohort), three patients experienced a grade 2 ALT abnormality, and 20 patients experienced a grade 1 ALT abnormality. No grade 4 abnormalities were reported in either dosing schedule. Across all patients, two experienced a grade 3 AST abnormality (both day 1 and 8 dose schedules; 50- and 100-mg cohorts), eight experienced a grade 2 AST abnormality, and 32 experienced a grade 1 AST abnormality. Few incidences of abnormal bilirubin levels were reported: in total, nine patients recorded abnormal bilirubin levels, with two patients experiencing grade 2 and two experiencing grade 3 events. Few of these events were reported to be treatment related.

Adverse events leading to patient discontinuation were reported in 13 patients; five cases were treatment related. Three patients treated in the day 1 and 8 dosing schedules (one patient, 125-mg cohort; two patients, 150-mg cohort) discontinued because of treatment-related neutropenia (grade 3-4), whereas one patient receiving 150 mg in the day 1/24-hour dosing schedule discontinued because of treatment-related thrombocytopenia (grade 4). One patient receiving 150 mg in the day 1/24-hour dose schedule experienced a nonhematologic adverse event (grade 2 phlebitis) during their first infusion, which led to their discontinuation.

Serious adverse events were reported during the on-treatment period in 20 patients (46%) in the day 1 and 8 dosing schedules and in nine patients (35%) in the day 1/24-hour dosing schedule. Four patients experienced treatment-related serious adverse events: in the day 1 and 8 dosing schedule, one patient treated with 100 mg BI 2536 was hospitalized because of treatment-related grade 4 neutropenia, one patient treated with 100 mg BI 2536 was hospitalized because of treatment-related grade 4 neutropenia and leucopenia, and one patient was hospitalized because of treatment-related grade 3 anemia, grade 4 leucopenia, grade 4 neutropenia, and grade 4 thrombocytopenia. In the day 1/24-hour dosing schedule, one patient experienced treatment-related grade 4 neutropenia.

Ten patients died as a result of adverse events that began during the on-treatment period; three of these patients died because of disease progression that was reported as an adverse event. One patient died of pneumonia and another patient died as a result of dyspnea. The other five patients died following a general deterioration of their physical condition. None of these fatalities were considered to be drug related.

Response data are summarized in Table 6. Thirteen patients (29%) receiving the day 1 and 8 dosing schedule were not evaluable according to RECIST, and 17 patients (65%) receiving the day 1/24-hour dosing schedule were not evaluable according to RECIST or had missing scans. In the day 1 and 8 dosing schedule, 14 patients (32%) had a best overall response of stable disease. No CRs or PRs were observed. Five patients (11%) showed no disease progression 3 months after initiating BI 2536 treatment; of these, one patient was in the 25-mg treatment cohort, and four patients were treated with the MTD (BI 2536 100 mg). These five patients had head and neck cancer (n = 1), breast cancer (n = 1), kidney and urothelial cancer (n = 1), and pancreatic cancer (n = 2). Of 24 patients treated with the MTD, six patients (25%) had stable disease, and 10 patients (42%) had disease progression as their best response; the remaining eight were not evaluable. No patients treated with BI 2536 in the day 1/24-hour dosing schedule recorded a CR or PR. Three patients, with ovary and fallopian tube, prostate, and small cell lung cancer, respectively, were reported to have no disease progression for 3 months after starting treatment.

BI 2536 administered as a 1-hour infusion at doses of 25, 50, 100, 125, 150, and 200 mg on day 1 and day 8 or as a 24-hour infusion at doses of 50, 100, 150, 175, 200, and 225 mg on day 1 of each treatment cycle showed a dose-proportional increase in exposure. Geometric mean drug plasma concentration-time profiles of BI 2536 administered as a 1-hour infusion are compared in Supplementary Fig. S1. BI 2536 exhibited multicompartmental pharmacokinetic behavior, with a fast distribution phase after the end of infusion followed by slower elimination phases. The volume of distribution was high, indicating a high distribution into tissues and organs, and a terminal elimination half-life of up to 50 hours was observed. Overall total plasma clearance of BI 2536 was high. No relevant accumulation of BI 2536 exposure was observed when two doses of BI 2536 were given on days 1 and 8 in a 3-week treatment cycle. Furthermore, no significant deviation from dose proportionality in BI 2536 exposure (AUC_0-∞ and C_max) was observed. BI 2536 pharmacokinetic variables following a 1-hour infusion are summarized in Supplementary Table S7. These could not be evaluated for the 50 mg treatment group on day 1 because of insufficient data or unreasonably high BI 2536 plasma concentrations. In addition, descriptive statistics of the 125- and 200-mg doses could not be evaluated on day 8 because patient data were available. Only 15 of 26 patients receiving 24-hour infusions had evaluable plasma concentration-time profiles permitting accurate statistical evaluations. Therefore, pharmacokinetic evaluations for this treatment schedule are not shown.

This phase I, open-label, dose-escalation study was designed to determine the MTD, safety, efficacy, and pharmacokinetics of two BI 2536 dosing schedules in patients with advanced solid tumors. The MTD of BI 2536 when administered as a 1-hour infusion on days 1 and 8 of a 21-day treatment course was determined as 100 mg per administration, that is, 200 mg per course. The MTD for the second dosing schedule, in which BI 2536 was administered as a 24-hour infusion on day 1 of a 21-day treatment course, was not determined. In this second treatment schedule, 225 mg was shown to be well tolerated, but the dose was not escalated further. No formal comparisons were made between BI 2536 dose levels or between the two dose schedules.

BI 2536 showed modest antitumor activity. Fourteen patients (32%) treated in the day 1 and 8 dosing schedules had a best overall response of stable disease. No CRs or PRs were observed in any treated patients. No patients treated with BI 2536 in the second dosing schedule recorded a response or stable disease in this heavily pretreated population, but 17 patients receiving this treatment schedule were not evaluable by RECIST criteria. Measurable disease as per RECIST was not an entry criterion into this trial. ln addition, there were a significant number of patients who were not evaluable according to RECIST because of early progression as indicated by a median total observation time of 47 days (range, 12-196 d) for patients receiving a 1-hour infusion and 47 days (range, 8-306 d) for those receiving a 24-hour infusion. Scans may not have been completed or followed up for patients who stopped the study early.

Plasma concentrations of BI 2536 increased in a dose-proportional manner and no relevant accumulation of exposure was observed when BI 2536 was administered on days 1 and 8 of a 3-week treatment cycle. The terminal elimination half-life was around 50 hours. BI 2536 was distributed fast into tissue as seen by the decline in plasma concentration after the end of infusion and the high volume of distribution.

The most frequently reported treatment-related nonhematologic adverse events in this study were gastrointestinal events and fatigue. Although neutrophil abnormalities were frequent and reached grade 3 to 4, these changes were reversible and did not result in a significant rate of infectious complications. Because BI 2536 induces cell-cycle arrest and apoptosis, systemic effects, such as hematotoxicity, cellular depletion of the bone marrow, and gastrointestinal effects, may be explained by the pharmacologic activity of the drug. Although no pharmacodynamic endpoints were analyzed as part of this study, neutropenia is a characteristic side effect observed with mitotic inhibitors and may be seen as a surrogate for target inhibition by BI 2536 (12).

Clinical investigation of established antimitotic therapies targeting the cell cycle (such as tubulin stabilizers or destabilizers) report neurologic and hematologic toxicities as the principle adverse events (13–15). As expected given the Plk-1–specific mode of action, relevant neurologic side effects have not been observed for BI 2536.

In parallel to this study, BI 2536 has also been investigated in a phase I repeated dose-escalation study in patients with advanced solid tumors (10). BI 2536 was administered as a single i.v. infusion every 21 days or daily infusions for 3 days every 21 days (10, 16). Doses in the range of 25 to 250 mg were investigated across both dosing schedules. In the single i.v. infusion schedule, the MTD was established as BI 2536 200 mg (10). For the consecutive day dosing schedule, the MTD was established as 60 mg/d. The main DLTs were neutropenia and febrile neutropenia; mild gastrointestinal toxicities were also observed. One patient with head and neck cancer experienced a transient PR (10). The total dose per schedule was similar for all treatment schedules investigated in the range of around 200 mg.

These first two clinical studies investigated the tolerability of four treatment regimens for BI 2536. The cumulative dose of BI 2536 administered during the first 3 weeks of either schedule consistently ranged between 180 to 225 mg, with similar side-effect and pharmacokinetic profiles. Hematotoxicity, which can be considered as a pharmacodynamic effect, was similar between all schedules. Based on phase I safety data and practical considerations, in particular, of the potential future combination of this or a similar agent with other anticancer agents, two schedules (a single i.v. infusion every 21 d and daily infusions for 3 d every 21 d) were chosen to be taken to phase II development.

BI 2536 is the first selective member of the Plk-1 inhibitor class that entered into clinical trials. Since then, a number of other Plk-1 inhibitors have been or are undergoing clinical investigation. One such agent is BI 6727, a second generation dihydropteridinone derivative with improved pharmacokinetic characteristics, which is currently in phase II clinical development.

In summary, these data show an adequate safety profile for BI 2536 in patients with advanced solid tumors. Findings from this study support further development of the Plk-1 inhibitor class for the treatment of advanced solid tumors.

H. Fritsch, V.L. Reichardt, D. Trommeshauser, and G. Munzert are employed at Boehringer Ingelheim.

We thank the editorial assistance of Ogilvy Healthworld.

Grant Support: Boehringer Ingelheim (A. Hochhaus, S-E. Al-Batran, and E. Jäger).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.