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. Author manuscript; available in PMC: 2016 Apr 18.
Published in final edited form as: Leuk Lymphoma. 2015 Mar 11;56(10):2834–2840. doi: 10.3109/10428194.2015.1014368

Phase I Dose Escalation Trial of the Novel Proteasome Inhibitor Carfilzomib in Patients with Relapsed Chronic Lymphocytic Leukemia and Small Lymphocytic Lymphoma

Farrukh T Awan 1, Joseph M Flynn 1, Jeffrey A Jones 1, Leslie A Andritsos 1, Kami J Maddocks 1, Ellen J Sass 1, Margaret S Lucas 1, Weihong Chase 1, Sharon Waymer 1, Yonghua Ling 1, Yao Jiang 1, Mitch A Phelps 1, John C Byrd 1, David M Lucas 1, Jennifer A Woyach 1
PMCID: PMC4835239  NIHMSID: NIHMS753631  PMID: 25669927

Abstract

The proteasome complex degrades proteins involved in a variety of cellular processes and is a powerful therapeutic target in several malignancies. Carfilzomib is a potent proteasome inhibitor which induces rapid CLL cell apoptosis in vitro. We conducted a phase I dose-escalation trial to determine safety and tolerability of carfilzomib in relapsed/refractory CLL or small lymphocytic lymphoma (SLL). Nineteen patients were treated with carfilzomib initially at 20 mg/m2, then escalated in 4 cohorts (27, 36, 45 and 56 mg/m2) on days 1, 2, 8, 9, 15, and 16 of 28-day cycles. Therapy was generally well-tolerated, and no dose-limiting-toxicities were observed. The most common hematologic toxicities were thrombocytopenia and neutropenia. All patients evaluable for response had stable disease, including patients with del17p13 and fludarabine-resistant disease. This trial shows acceptable tolerability and limited preliminary efficacy of carfilzomib in CLL and SLL.

Keywords: Proteasome inhibitor, Chronic Lymphocytic Leukemia, carfilzomib

Introduction

Therapy for chronic lymphocytic leukemia (CLL) has advanced significantly over the past decade, with the introduction of novel targeted therapies demonstrating improved disease and symptom control(1-5). However, cure remains elusive and there continues to be a need to identify and target novel pathways in the cancer cell to effect deeper and more sustained remissions.

The proteasome is a multicatalytic intracellular enzyme complex that serves to degrade intracellular proteins through a complex and selective mechanism involving capture and unfolding of polyubiquitinated proteins which subsequently undergo proteolysis by specific proteasomal peptidases(6-8). Proteins involved in a wide variety of cellular functions are degraded by the proteasome; this includes proteins involved in cell cycle regulation (e.g. CDKs), apoptosis (e.g. p53, Mcl-1 and Bax) and transcription factors (e.g. NF-κB). Because of its role in these key cancer pathways, the proteasome plays an important role in tumor cell development and survival and is thus an important target for cancer therapy(6-9).

Bortezomib was the first proteasome inhibitor in clinical use and is currently approved to treat multiple myeloma and mantle cell lymphoma(10, 11). Based on its efficacy in multiple myeloma, it was postulated that bortezomib would be efficacious in patients with CLL. However, despite promising preclinical studies showing induction of apoptosis in CLL cells alone(12) and in combination with nucleoside analogs(13, 14), clinical trials in CLL failed to show significant efficacy(15). This was partly attributed to the inhibition of bortezomib by dietary flavonoids in human plasma, specifically quercetin, which is present at significant concentrations in patients with CLL(16).

Carfilzomib is a synthetic tetrapeptide ketoepoxide-based small molecule(17-19). It functions as a specific inhibitor of the chymotrypsin-like activity of the 20S proteasome, leading to the accumulation of protein substrates within the cell and induction of apoptosis(18-22). Unlike bortezomib, carfilzomib is an irreversible proteasome inhibitor, but similar to bortezomib, carfilzomib-inhibited proteasome activity has been shown to recover within approximately 24 hrs, most likely due to new proteasome synthesis.(19, 23) Carfilzomib was also shown to more potently induce apoptosis of CLL cells compared to bortezomib, and unlike bortezomib, was equally effective in media with human versus fetal bovine serum.(24) Furthermore, carfilzomib can induce cell death in bortezomib-resistant cells.(22) One potential mechanism of cell death is through the NF-kB pathway, which is well-established to promote survival signaling in CLL.(25) CLL cells exposed to increasing doses of carfilzomib showed an accumulation of pIκB and decreased IκB in the cytoplasm, along with nuclear accumulation of NF-κB. Similarly, p53 and select downstream targets of p53 such as p21, NOXA and PUMA were also found to be consistently up-regulated in patient CLL cells(24).

Based on promising pre-clinical data and an acceptable toxicity profile reported from previous early phase studies in hematologic malignancies, we conducted a phase I clinical trial of carfilzomib in patients with relapsed CLL.

Materials and methods

Adult patients (≥18 years old) with symptomatic, previously treated CLL were enrolled from November 2010 to April 2013 after obtaining written informed consent. Patients were eligible if they had histologically confirmed CLL or small lymphocytic lymphoma (SLL) and required therapy per International Workshop on Chronic Lymphocytic Leukemia (IWCLL) 2008 criteria(26). All patients were also required to have adequate organ function, defined as creatinine clearance (CrCl) >15mL/min, alanine aminotransferase (ALT) <3 times the upper limit of normal (ULN), bilirubin ≤2 times the ULN unless disease related; platelets >20 × 109/L, and absence of active bleeding. All patients had an ECOG performance status ≤2 and the absence of known other malignancies that could result in a life expectancy of <2 years or that would have confounded assessment of toxicity in the study. The trial was registered at www.clinicaltrials.gov with the identifier NCT01212380.

Pretreatment And Serial Laboratory Assessments

Baseline laboratory assessments included complete blood count with differential, platelet count, absolute lymphocyte count (ALC); serum chemistries including liver functions (comprehensive metabolic panel - CMP); quantitative immunoglobulins, uric acid, lactate dehydrogenase (LDH), bone marrow aspiration and biopsy with standard and interphase cytogenetics via fluorescence in situ hybridization (FISH). Patient samples were collected weekly for CBC and CMP assessment at the beginning of every cycle. Patients also were assessed for tumor lysis by measuring phosphorus, uric acid and LDH at baseline and 4 hours after each dose, and serum potassium levels were assessed at 2, 4, 6 hours post-dose. Samples for pharmacokinetic and pharmacodynamic studies were collected at baseline and on days 1, 2 and 8 of cycle 1.

Treatment

Carfilzomib was administered as a 30-minute intravenous (IV) infusion at a specified dose in mg/m2 daily for 2 days every week for 3 weeks (days 1, 2, 8, 9, 15, and 16). The first and second doses of cycle 1 were always administered at 20 mg/m2. Starting at cycle 1 day 8, carfilzomib was dosed at 27 mg/m2 and escalated with every cohort to 36, 45 and 56 mg/m2. Therapy was administered in 28 day cycles and continued until disease progression or intolerance for up to 12 cycles. All patients were pre-medicated with dexamethasone (4 mg, orally or IV) at least 2 hrs prior to carfilzomib dosing during the first cycle, and at the investigator's discretion in subsequent cycles.

Toxicity Assessment And Dose-Limiting Toxicity

Subjects were evaluated for toxicity according to the Common Terminology Criteria for Adverse Events (CTCAE) of the NCI version 3.0 for non-hematologic toxicities and using the IWCLL 2008 criteria for hematologic toxicities. Dose limiting toxicity (DLT) was defined as any of the following events with at least possible relationship to carfilzomib occurring during the first cycle of therapy: grade ≥ 2 neuropathy with pain, any grade ≥3 toxicity (excluding self-limited nausea, vomiting, diarrhea), grade ≥3 nausea, vomiting, or diarrhea after 3 days of maximal antiemetic/antidiarrheal therapy, grade ≥4 fatigue lasting for ≥7 days, tumor lysis syndrome requiring dialysis, any non-hematologic toxicity requiring a dose reduction during cycle 1, and inability to receive day 1 dose of cycle 2 due to drug-related toxicity persisting from cycle 1 or drug-related toxicity newly encountered on day 1 of cycle 2 with a delay of >21 days. Hematologic DLTs included grade 4 neutropenia lasting for ≥7 days in patients with pre-treatment ANC >1.0 × 109/L, febrile neutropenia (ANC <1.0 × 109/L with a fever ≥ 38.3°C) in patients with pre-treatment ANC >1.0 × 109/L, grade 4 thrombocytopenia (platelets <20 × 109/L) at day 1 of cycle 2, grade ≥3 thrombocytopenia associated with clinically significant bleeding, any hematologic toxicity requiring a dose reduction within cycle 1, and inability to receive day 1 dose of cycle 2 due to drug-related toxicity persisting from cycle 1. Follow-up safety visits occurred weekly for the duration of the study.

Criteria For Dose Escalation

Dose escalation was based on standard 3+3 design for phase I studies. Three patients were treated at a given dose level, beginning at dose level (DL) 0 (27 mg/m2). Escalation to the next dose level occurred when no patients experienced a DLT during cycle 1. The maximum tolerated dose (MTD) was defined as the dose at which <1/3 or <2/6 patients exhibited a DLT.

Response Assessments

Although response was not the primary endpoint of this trial, all patients were evaluated for response utilizing the IWCLL 2008 criteria. Initial response assessment, including clinical assessment and CT scans, occurred following cycle 2. Thereafter, clinical assessment for response occurred every 2 months, with CT scans and bone marrow biopsies being performed if other evidence supported a complete response.

Pharmacokinetic Studies

Patient plasma samples were collected prior to dosing, 15 and 25 minutes (mins) during the infusion, and 5, 10, 30, 45, 60, 90, 120 and 240 mins post-dose on cycle 1 day 1 (C1D1) and cycle 1 day 8 (C1D8). Carfilzomib was quantified by a validated liquid chromatography-mass spectrometry (LC-MS) assay. Carfilzomib and deuterated internal standard, d10-carfilzomib, were provided by the National Cancer Institute. Human plasma was purchased from the American Red Cross (Columbus, OH). An E-pure water purification system (Barnstead, Dubuque, IA) was used to obtain HPLC grade water (>18 mΩ). All organic solvents were of HPLC grade and were purchased from Fisher Scientific (Pittsburgh, PA). Reagent grade formic acid was purchased from Sigma (St. Louis, MO). The LC-MS system consisted of a Finnigan TSQ Quantum EMR triple quadrupole mass spectrometer (Thermo Fisher Scientific Corporation, Sunnyvale, CA) and a Dionex Ultimate 3000 HPLC system (Thermo Fisher Scientific Corporation, Sunnyvale, CA). Both carfilzomib and internal standard were separated on a Betabasic-C8 column (Thermo Scientific Corp, 50x2.1mm ID, 5μm) by gradient elution with binary mobile phases consisting of water and methanol with 0.1% formic acid. Parent and product ion pairs, 720.63>402.29 and 730.68>412.36, were selected to monitor carfilzomib and d10-carfilzomib in multiple reaction monitoring (MRM) mode, respectively. The plasma concentrations of carfilzomib were calculated against a calibration curve of carfilzomib spiked in human plasma prior to extraction with ethyl acetate. The dynamic range of the calibration curve of carfilzomib was from 0.1 ng/mL to 1000 ng/mL with regression coefficient (r2) exceeding 0.99. Sample analysis was accompanied by quality controls with acceptable precision and accuracy values. Pharmacokinetic analysis was performed using non-compartmental methods in Phoenix WinNonlin 6.3.0.395 (Pharsight Corporation, Mountain View, CA, USA).

Pharmacodynamic Studies

A validated proteasome activity assay that measures chymotrypsin-like activity of CLL cells from patient samples was performed using commercially available reagents (EMD Millipore, Billerica, MA). Proteasome inhibition in CLL cells was examined in samples obtained at screening, pre-treatment and post-treatment days 1, pre-treatment day 2, pre and post-treatment days 8, and pre and post-treatment day 9.

Statistical Methods

The primary endpoint of the study was to define the MTD of carfilzomib, using the traditional 3+3 phase I design as described above. Secondary endpoints included describing response rates and conducting pharmacokinetic and pharmacodynamic studies following administration of carfilzomib. Common hematologic and non-hematologic toxicities are summarized with frequencies by dose level and for all patients. Baseline characteristics are described using frequencies and medians with ranges for categorical and continuous variables, respectively. Responses are summarized in the results section. Observed pharmacokinetics parameters were evaluated and summarized in graphic and table format. No inferential statistical tests of hypotheses were planned due to the small sample size.

Results

Patient Characteristics

Nineteen patients with previously treated, relapsed CLL were enrolled in a sequential 3+3 manner on this phase I trial (Table 1) between November 2010 and April 2013. Median patient age was 70 years, and 95% were males. The patients were very high risk: 47% of patients had del(17p13.1), 21% had del(11q22.3), 70% had complex karyotype, 84% had IGHV unmutated disease, and 68% were refractory to fludarabine. 47% of patients were refractory to their most recent therapy (Table 2). Three of the patients were refractory to ibrutinib; one patient did not have sample available for mutational testing but the other two patients had known mutations in PLCγ2 at R665W, and the other had mutations in both BTK (C481S) and PLCγ2 (R665W, L845F, S707Y).(27)

Table 1.

Study Design

Cohort Dosing Schedule No. of patients
0 20 mg/m2 on days 1 and 2 and 27 mg/m2 on days 8, 9, 15 and 16 of every cycle. Cycles were repeated every 28 days 5
1 20 mg/m2 on days 1 and 2 and 36 mg/m2 on days 8, 9, 15 and 16 of every cycle. Cycles were repeated every 28 days 4
2 20 mg/m2 on days 1 and 2 and 45 mg/m2 on days 8, 9, 15 and 16 of every cycle. Cycles were repeated every 28 day 4
3 20 mg/m2 on days 1 and 2 and 56 mg/m2 on days 8, 9, 15 and 16 of every cycle. Cycles were repeated every 28 days 6
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Table 2.

Patient demographics (n = 19)

Patient Characteristics
Age in years, median (range) 70 (43-86)
≥ 65 years, n (%) 11 (57)
Female, n (%) 1 (5)
Rai stage at study entry, n (%)
Intermediate Risk (I/II) 3 (15)
High Risk (III/IV) 16 (84)
WHO Performance Status, n (%)
0 5 (26)
1 14 (73)
2 0
Organomegaly, n (%)
Radiographic evidence of splenomegaly (>10 cm) 11 (57)
Radiographic evidence of hepatomegaly (>12cm) 3 (15)
Lymphadenopathy 16 (84)
Hematologic parameters, median (range)
WBC (×103/μl) 9.8 (2.3-211)
Hemoglobin (g/dL) 10.3 (7.7-13.5)
Platelets (× 109/L) 67 (20-156)
LDH (μg/mL), median (range) 207 (93-526)
Unmutated IGHV, n (%) 16 (84)
Interphase cytogenetic abnormalities, n (%)
n (%) with del(13q14.3) 7 (36)
n (%) with del(11q22.3) 4 (21)
n (%) with del (17p13.1) 9 (47)
n (%) with Trisomy 12 3 (15)
Treatment history, n (%)
Prior therapies, median (range) 3 (1-8)
n (%) alkylator-refractory 16 (84)
n (%) fludarabine-refractory 13 (68)
Weeks since most recent therapy, median (range) 4 (2-42)
Refractory to most recent therapy, n (%) 9 (47)
Refractory to ibrutinib, n (%) 2 (10)
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Toxicity

Carfilzomib was generally well tolerated, and no DLTs were observed. The most common hematologic toxicities were thrombocytopenia (grade 3, n=3; grade 4, n=2) and neutropenia (grade 3, n=4; grade 4, n=2). Significant (grade 3 or 4) non-hematologic toxicities included grade 3 dyspnea (n=1) and grade 3 febrile neutropenia (n=1). Electrolyte disturbances were seen more frequently, but all were grade ≤2. No patient experienced an infusion reaction or tumor lysis syndrome. Cardiac and hepatic side effects were also not common; only one patient each experienced sinus tachycardia (grade 1) and transaminitis (grade 1), respectively (Table 3).

Table 3.

Incidence of all study-related adverse events by system organ class, by preferred term, and by grade (n = number of patients; total n=19)

Grades
1 2 3 4
Blood and lymphatic system disorders
Anemia 7 2 1
Neutropenia 4 2
Thrombocytopenia 4 4 3 2
Gastrointestinal
Nausea 1
Increased aspartate aminotransferase 1
Hypoalbuminemia 1 1
Electrolyte imbalance
Hypocalcemia 4 3
Hyperkalemia 2
Hypokalemia 4
Hypophosphatemia 1 1
Hypernatremia 2
Hyponatremia 2
Renal
Renal failure (elevated Cr) 2
Hyperuricemia 1
Others
Dyspnea 1
Febrile neutropenia 1
Sinus tachycardia 1
Peripheral neuropathy 1
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Therapy Received and Response

Three patients were treated for at least 1 cycle at each dose level, and 6 patients were treated at the maximum evaluated dose of 56 mg/m2. Patients received a median of 2 cycles of therapy (range 1-12). Four patients discontinued therapy during cycle 1; two at a dose of 27 mg/m2 discontinued for disease progression, one at a dose of 36 mg/m2 discontinued due to a comorbid medical condition, and one patient treated at 45 mg/m2 discontinued due to patient decision. Four patients completed only one cycle; two discontinued due to disease progression and two because of cytopenias related to disease.

Eleven patients were evaluable for response assessment at the end of cycle 2 and all had stable disease. This included 5 patients with del(17p13.1) and 8 patients with fludarabine-resistant disease. Two patients who previously received ibrutinib discontinued therapy after one month or less of ibrutinib, but one was treated for 4 months before progression was detected in the central nervous system. Two patients received over 6 cycles of therapy; one received the planned 12 cycles of therapy and had stable disease. One patient received 7 cycles of therapy and achieved a 62% reduction of lymphadenopathy and normalization of counts after 6 cycles; but this could not be reevaluated after 2 months to be consistent with a partial response as per the IWCLL 2008 criteria, since this patient discontinued therapy due to lifestyle considerations with frequent infusions.

Pharmacokinetics

Samples from all 19 patients were available for analysis at the initial dose of 20 mg/m2 at C1D1 and for subsets of patients at higher doses at C1D8. Concentration-time curves are shown in Figure 1. PK parameter estimates based on non-compartmental analysis are summarized for all patients for different days and dose levels (Table 4). Maximum observed concentration (Cmax) on Cycle 1 day 8 ranged from 0.6 μg/mL to 7.9 μg/mL across the dosing cohorts, and area under the observed concentration vs. time curve (AUC) ranged from 234 hr*ng/mL to 1833 hr*ng/mL. The Cmax and AUC for one patient treated at the 56 mg/m2 dose level on day 8 were approximately 5 to 20-fold higher relative to other patients treated at this dose level. However, no remarkable response or toxicities were observed with this patient.

Figure 1. Plasma concentration vs. time plots of carfilzomib.

Figure 1

Figure 1

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A. Carfilzomib concentrations in plasma were measured using a validated LC/MS-MS assay. Data are shown for all nineteen patients at C1D1 at the initial dose of 20 mg/m2; B. Data shown for subsets of patients at C1D8 at 27, 36, 45, and 56 mg/m2.

Table 4.

Non-compartmental pharmacokinetic parameter estimates

Carfilzomib Pharmacokinetic parameters summary
Cycle 1 Day 1 Cycle 1 Day 8
Dose=20 mg/m2 (n=19) Dose=27 mg/m2 (n=4) Dose=36 mg/m2 (n=4) Dose=45 mg/m2 (n=3) Dose=56 mg/m2 (n=5)*
Parameters Mean ±SD Mean ±SD Mean ±SD Mean ±SD Mean ±SD
AUCall (hr*ng/mL) 247± 150 234± 71.6 360± 106 360± 34.5 785± 265
T½ (hr) 1.4± 1.6 0.7± 0.4 0.6± 0.4 1.1± 0.6 1.0± 0.2
Cmax (ng/mL) 559± 227 591± 196 918± 330 932± 73.0 1653± 112
CL (L/hr/m2) 95.9± 33.9 123± 35.6 107± 30.2 126± 12.6 78.4± 27.4
Vss (L/m2) 23.8± 37.4 16.3± 7.2 14.5± 9.6 11.6± 1.1 11.0± 4.7
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*

Data from one patient is excluded due to significantly higher concentrations measured in samples from that patient compared to the other 5 patients.

AUCall: area under the observed plasma concentration-time curve

T½: half-life

hr: hour

Cmax: maximum observed concentration

CL: clearance

Vss: volume of distribution at steady state

Pharmacodynamics

Seven patients had sufficient samples at baseline and at least one serial time point to evaluate proteasome activity. One patient each received 27, 36, and 45 mg/m2 carfilzomib, and three patients received 56 mg/m2. In all patients, regardless of dose, carfilzomib administration resulted in suppression of chymotrypsin-like proteasome activity one hour after dosing. This activity partially recovered by 24 hours, and appeared in fact to rebound higher than pretreatment levels by day 8 (Figure 2).

Figure 2. Target inhibition by carfilzomib.

Figure 2

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Chymotrypsin-like proteasome activity was measured in CLL cells from whole blood obtained at screening, pre-treatment, and post-treatment day 1, pre-treatment day 2, and pre and post-treatment day 8 using a commercial assay. Proteasome activity in each sample is shown relative to the activity in that patient's pretreatment sample.

Discussion

Our study describes the first phase I dose escalation study of carfilzomib in patients with relapsed or refractory CLL. In this study, infusions of carfilzomib at doses up to 56 mg/m2 twice weekly for 3 weeks of a 4 week cycle were well-tolerated. No maximum tolerated dose of carfilzomib was identified. No objective partial or complete responses were observed as defined by the IWCLL 2008 criteria(26). The incidence and severity of adverse events or infections were not related to the dose of carfilzomib administered. There was also evidence of myelosuppression, but this did not result in infectious morbidity.

Previous investigations showed that bortezomib produced no objective clinical responses in CLL(15). One putative mechanism for this lack of efficacy is the inhibition of bortezomib by dietary flavonoids in human plasma, specifically quercetin (16). While this inhibitory effect is present in myeloma cells as well, higher levels of quercetin are required to inhibit apoptosis (40 μm, vs. 20 μm in CLL cells)(16). This was determined not to be an issue with carfilzomib, as it lacks the boronate moiety that is the target of hydroxyl groups in some dietary flavonoids(29). Our group previously reported that carfilzomib demonstrates significant cytotoxicity against CLL cells in vitro. This activity is superior to bortezomib, and consistent with the above findings, is unaffected by human serum(24). Given these preclinical results, and the efficacy and excellent tolerability observed in clinical trials of carfilzomib in patients with multiple myeloma(30), we decided to pursue this phase I clinical trial to determine the tolerability and preliminary efficacy of carfilzomib in patients with relapsed and refractory CLL.

Similar to results observed in patients with multiple myeloma(30), significant inter- and intra-patient variability was observed in carfilzomib exposures. To date, no other pharmacokinetic studies using a 30-min infusion of carfilzomib have been reported. Our pharmacokinetic estimates at 20 mg/m2 were similar to those recently reported with a 2-10 min infusion(31). The observed mean Cmax at 20 mg/m2 in our study (559 ± 227 ng/mL) was low compared to the other report (2390 ± 104 ng/mL). This is potentially due to the longer infusion duration in our study; however, both AUC and Cmax for this study were lower compared with reported data in patients receiving shorter infusions(32). In contrast, the observed Cmax values in our study were similar to those of two previous reports with IV bolus administration of carfilzomib(30, 33).

Importantly, we observed in all patients with evaluable samples a reduction in chymotrypsin-like proteasome activity, which recovered partially by 24 hrs and completely by day 8. These results demonstrated the on-target effects of carfilzomib on CLL B-cells. They also raise the question of whether more prolonged proteasome inhibition might be beneficial in this disease. An oral proteasome inhibitor would provide the ability to continuously inhibit the proteasome in this manner, although it would be important to be sure that unmanageable toxicities are not observed.

Clinical benefit with nodal responses was observed in some patients even at the lowest dose cohort, but these were not sufficient to be characterized as complete or partial responses. Similar to the study with bortezomib(15), we saw a trend toward prolonged stable disease in patients receiving higher doses of carfilzomib, which suggests some anti-tumor effect. One reason for the lack of responses observed could be the nature of this study group, with most patients having genetically high-risk (i.e. del(17p13.1), del(11q22.3), complex karyotype) disease.

Our study demonstrates the safety and tolerability of carfilzomib in the management of patients with relapsed/refractory CLL. While unlikely to move forward in this disease as a single agent given the minimal efficacy observed, carfilzomib may be interesting to pursue as part of combination strategies in which efficacy of another drug may be improved by inhibition of proteasome-mediated protein breakdown. The availability of oral proteasome inhibitors should make integration of combination therapy with other agents more feasible. The landscape of CLL therapy is changing dramatically with the approval of kinase inhibitors, but mechanistically new, well-tolerated and effective therapies that can be used in combination strategies will be required to achieve a cure.

Acknowledgements

The authors thank all of the patients who participated in this trial as well as the dedicated nurses, nurse practitioners and physicians who cared for them. This work was supported by Specialized Center of Research from the Leukemia and Lymphoma Society, P50-CA140158, the D. Warren Brown Foundation, the Conquer Cancer Foundation, and Onyx Pharmaceuticals, Inc, an Amgen subsidiary.

Footnotes

Conflict of Interest: The authors declare no other relevant conflicts of interest.

Registration number at ClinicalTrials.gov: NCT01212380

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