The Cyclin-Dependent Kinase Inhibitor Flavopiridol Potentiates Doxorubicin Efficacy in Advanced Sarcomas: Preclinical Investigations and Results of a Phase I Dose-Escalation Clinical Trial

Cyclin-dependent kinases (CDK) are serine/threonine kinases that are activated upon association with cyclin proteins to form CDK complexes. In response to mitogenic and stress stimuli, CDK complexes phosphorylate effector protein complexes, regulating both RNA polymerase II–mediated transcription and progression through the cell cycle. CDKs are attractive targets for drug development, given that certain malignancies are dependent on dysregulated cyclin activity (1) and CDK inhibition has been observed as a potent vehicle to overcome resistance to standard chemotherapy (2–4).

Flavopiridol is a pan-CDK inhibitor that specifically inhibits CDK2, CDK4, CDK6, and CDK9 at nanomolar concentrations. Inhibition of CDKs by flavopiridol blocks cell-cycle progression at the G₁–S or G₂–M checkpoints and is associated with cell-cycle arrest and subsequent apoptosis (5, 6). Flavopiridol has been tested at various dosing levels and schedules in both hematologic (7, 8) and solid tumor malignancies (3, 9, 10). To date, the most compelling data about clinical efficacy have been observed in hematologic malignancies, such as chronic lymphocytic leukemia (11).

The therapeutic impact of single-agent flavopiridol in solid tumors has been less robust. However, preclinical models and clinical trial results suggest a use of flavopiridol in combination with chemotherapy. For example, flavopiridol, as a pan-CDK inhibitor, blocks CDK9 leading to suppression of Rad51, an enzyme involved in homologous recombination. This defect in DNA repair then sensitizes tumor cells to p53-dependent induction of apoptosis by irinotecan (12). Flavopiridol has also been associated with the potentiation of other chemotherapies such as paclitaxel, docetaxel, gemcitabine, and doxorubicin, in which flavopiridol exposure has been shown to enhance apoptosis in tumor cells (13–17).

Advanced sarcomas are a group of heterogeneous mesenchymally derived neoplasms for which clinical treatment options are limited. After progression to metastatic or unresectable disease, systemic treatment options include chemotherapies such as doxorubicin, ifosfamide, dacarbazine, gemcitabine, and docetaxel. While the molecular characteristics of these diseases are increasingly being elucidated, cytotoxic chemotherapy remains the clinical standard-of-care. Unfortunately, response rates to these agents are low and patients rarely obtain durable clinical benefit.

Preclinically, flavopiridol has been shown to potentiate the effects of doxorubicin in a bone sarcoma (osteosarcoma), especially in a retinoblastoma-null background (18). However, the ability of flavopiridol to potentiate doxorubicin in soft tissue sarcoma is unknown. Certain soft tissue sarcoma subtypes, such as well-differentiated and dedifferentiated (WD and DD) liposarcoma, are especially attractive for drug targeting by flavopiridol, as CDK4 is amplified in 90% of these tumors (19). WD and DD liposarcoma has been shown to be sensitive to inhibition of CDKs in preclinical models, and early-phase clinical trials have hinted that targeted inhibition of CDKs may lead to impressive clinical benefit (20). Therefore, we evaluated both the in vitro and in vivo ability of flavopiridol to potentiate the effects of doxorubicin in sarcoma and conducted a phase I clinical trial of the combination in patients with advanced sarcomas.

LS141 primary human cell line was derived from a patient with high-grade retroperitoneal dedifferentiated liposarcoma and the malignant peripheral nerve sheath tumor (MPNST) cells were derived from a patient with a high-grade peripheral nerve sheath tumor of the thigh (graciously supplied by Jonathan Fletcher, Dana-Farber Cancer Institute, Boston, MA). These were grown in RPMI-1640 supplemented with 15% heat-inactivated FBS plus penicillin and streptomycin.

MPNST cells were treated with doxorubicin, flavopiridol (graciously supplied by National Cancer Institute, Bethesda, MD), or the combination of the 2 drugs together in sequence. MPNST cells were chosen given that LS141 (and other CDK4-dependent) cells are exquisitely sensitive to CDK4 inhibition in vitro, thus making combination studies uninterruptible. MPNST cells were plated, in triplicate, at a density of 1,000 cells per 100 mm² per plate. Twenty-four hours after plating, cells were treated for 24 hours with the IC₅₀ of doxorubicin (D, 15 nmol/L), flavopiridol (F, 150 nmol/L), drug-free media (control), or a combination of the 2 drugs, either concomitantly or sequentially for 24 hours each. After treatment, drug-containing medium was removed and cells were allowed to grow for 10 days to form colonies. The resulting colonies were stained with 0.01% crystal violet for 30 minutes and colonies counted using an automated colony counter (ColCount, Oxford Optronix). Results are presented as the percentage of untreated controls, and the statistical significance of the experimental results was determined by the 2-sided t test.

MPNST cells were lysed in radioimmunoprecipitation assay (RIPA) buffer supplemented with protease inhibitor cocktail tablets (Complete Mini, Roche Diagnostics) and 1 mmol/L NaVO₃. Total protein concentration of the lysates was measured by Bio-Rad protein assay (Bio-Rad Laboratories), and equal amounts of protein were loaded on 4% to 12% PAGE gels (Invitrogen). Polyvinylidene difluoride membranes were blocked with 5% nonfat dried milk in PBS buffer containing 0.1% Tween-20 (PBST) for 1 hour and probed with antibodies for full-length and cleaved PARP (Santa Cruz Biotechnology) and tubulin (Cell Signaling).

LS141 xenografts were established by directly implanting into severe combined immunodeficient (SCID) mice. Once tumors reached 100 mm³, groups of 5 mice were treated with the maximum tolerated dose (MTD) of flavopiridol (9 mg/kg), doxorubicin (0.9 mg/kg), or doxorubicin (0.7 mg/kg) followed by flavopiridol (7 mg/kg) at selected time points (1, 4, and 7 hours). In addition, one set of animals was treated in reverse order of flavopiridol followed by doxorubicin, administered 7 hours apart. All treatments were administered in intraperitoneal fashion, twice weekly, for a total of 5 treatments. Tumors were measured every 2 to 3 days with calipers, and tumor volumes were calculated by the formula π/6 × (large diameter) × (small diameter)². Tumor volume was compared between groups of mice at various points in time based on the experiment and the statistical significance of the experimental results was determined by the 2-sided t test. Given the aggressive morbidity of the tumors, animal survival data could not be estimated. Toxicity was monitored by weight loss. These studies were done in accordance with the Principles of Laboratory Animal Care (NIH Publication No. 85-23, released 1985), under an Institutional Animal Care and Use Committee (IACUC)-approved protocol.

All patients (age ≥ 18, male, and female) were histologically confirmed to have metastatic or locally recurrent sarcoma. Prior treatment excluded anthracyclines but allowed up to 2 prior lines of therapy. PPAR-γ agonists, thalidomide or targeted therapy such as tyrosine kinase inhibitors, were not considered as prior lines of therapy. A minimum of 3 weeks from last treatment had to elapse before study entry (6 weeks for nitrosoureas and mitomycin C and 1 week for targeted agents). Patients had to have a Karnofsky performance status ≥ 60%, total white blood cell count ≥ 3,500/mm³, absolute neutrophil count (ANC) ≥ 1,500/mm³, platelets ≥ 100,000/mm³, and adequate hepatic, renal, and cardiac function (including ejection fraction ≥ 50%). Patients with central nervous system metastases were not eligible. Patients with a history of significant heart disease or radiation to both the pelvis and spine were additionally excluded. The protocol was approved by the Institutional Review Board of Memorial Sloan-Kettering Cancer Center (New York, NY) and all patients provided written informed consent.

The trial design was a nonrandomized, open-label dose-escalation of doxorubicin and flavopiridol. Groups of 3 to 6 patients were treated sequentially according to the dose-escalation in Table 1. Treatment was every 3 weeks (1 cycle), provided the ANC was ≥1,500/mm³ and platelet count ≥100,000/mm³. Doxorubicin was given intravenous) push over 5 to 7 minutes on day 1 of each cycle at a dose of 60 or 75 mg/m². On the basis of our and in vitro and in vivo preclinical model (see Results), flavopiridol was given 1 hour following doxorubicin as a 60-minute i.v. bolus (cohorts 1–6), starting at a dose of 40 mg/m² to a goal escalation dose of 70 mg/m², the approximate MTD defined in single-agent bolus schedule studies (21). This dose has also been shown to consistently achieve more than 2.0 μmol/L of flavopiridol in human plasma. In view of 90% protein binding in plasma, this achieves a therapeutically active free flavopiridol plasma level of approximately 200 nmol/L. Given the desire to continue to increase flavopiridol exposure and the success of split dosing (bolus followed by infusion) in the treatment of chronic lymphocytic leukemia (22), further cohorts were examined using a split dosing schedule. Patients in cohorts 7 and 8 received flavopiridol as a 30-minute bolus followed by a 4-hour infusion on day 1 of each cycle, beginning 1 hour after the administration of doxorubicin. The target flavopiridol dose was 90 mg/m² (Table 1); the single-agent MTD with divided dose flavopiridol therapy. Because of concerns for tumor lysis syndrome with the split-dose schedule, tumor lysis blood samples were obtained, including lactate dehydrogenase, calcium, magnesium, and phosphorous, on the day following therapy. Where indicated, dexrazoxane was given before each dose of doxorubicin (cumulative doxorubicin dose > 300 mg/m²). Dexrazoxane was given at 10 times the dose of doxorubicin. Doxorubicin was given within 30 minutes of start of the dexrazoxane infusion. After 600 mg/m² doxorubicin (including use of dexrazoxane), doxorubicin was discontinued and flavopiridol could be continued as a single agent until progression of disease. All treatments were administered in the outpatient setting and intrapatient dose-escalation was not permitted.

Toxicity was graded in accordance with the Common Toxicity Criteria Version 3.0 (23). Dose-limiting toxicity (DLT) was defined as the occurrence during the first cycle of grade IV hematologic toxicity 21 days after treatment, grade IV hematologic toxicity lasting 7 days or longer, grade III or IV nonhematologic toxicity including diarrhea despite antidiarrheal prophylaxis, nausea despite maximum antiemetic therapy, or any delay in treatment of more than 2 weeks. The MTD was defined as the dose one level below the dose at which 2 or more of the patients in the initial dose level experienced DLT during the first treatment course. Patients who experienced a DLT, or toxicity attributed to study medication, could continue to receive study treatment after recovery, with appropriate dose modifications as defined per protocol.

To be evaluable for response and to be assessable for determination of MTD, patients had to have received at least one full cycle of therapy. Otherwise, treatment responses were evaluated after every 2 cycles with computed tomographic scans or other diagnostic tests, as appropriate. Response Evaluation Criteria in Solid Tumors (RECIST), version 1.0, were used for response assessment and done by an independent protocol radiologist. Complete or partial responses (PR) were confirmed by repeat studies at least 4 weeks after the criteria for response were first met.

The main objective of this study was to determine the MTD or achieve a maximal identified target dose of flavopiridol, when administered in combination with a doxorubicin. Standard 3 + 3 design was used for dose-escalation. The incidence of hematologic and nonhematologic toxicities was summarized separately by flavopiridol cohort. Secondary analyses included a pharmacokinetic (PK) analysis of flavopiridol by noncompartmental methods.

Flavopiridol (also known as alvocidib, HMR-1275) was supplied by Sanofi Aventis Pharmaceuticals and distributed by CTEP. Doxorubicin and dexrazoxane are commercially available.

Blood samples for PK studies were collected into heparin-coated tubes and analyzed per previously published methods (24). Flavopiridol levels were measured on the bolus schedule before treatment, at completion of flavopiridol (time 0) and then 0.5, 1, 2, 6, and 24 hours later. For the split dosing schedule, PK samples were collected pretreatment and at approximately 0.5, 4.5, 24, and 28 hours from initiation of flavopiridol infusion.

In vitro, the doxorubicin and flavopiridol combinations were statistically superior to single-agent doxorubicin or flavopiridol. As shown in Fig. 1A, colony formation assays revealed that in comparison with doxorubicin or flavopiridol alone, observable colonies decreased significantly with all the combinations tested. For example, colony formation decreased from 59% with flavopiridol alone to 41% with concomitant therapy (combo, P = 0.046), 35% with doxorubicin followed by flavopiridol (D24, F24, P = 4.4 × 10⁻⁵), and 44% with flavopiridol followed by doxorubicin (F24, D24, P = 3.8 × 10⁻⁵). Although there was a trend in decrease of colony formation favoring doxorubicin followed by flavopiridol, no statistically significant differences in colony formation observed for the 3 combinations. However, when these combinations were examined for induction of apoptosis by PARP cleavage, this was predominantly observed for the cells treated with the sequential doxorubicin followed by flavopiridol combination (Fig. 1A).

In vivo, human xenograft mouse models revealed that single-agent flavopiridol decreased tumor growth relative to untreated controls and was significantly more efficacious than single-agent doxorubicin. As we have previously reported that the timing of chemotherapy relative to flavopiridol can affect the degree of tumor regressions in vivo, we elected to treat our tumor xenografts with doxorubicin followed by flavopiridol at 1, 4, and 7 hours, as well as with the reverse combination at a 7-hour interval. As shown in Fig. 1B, the only combination that was statistically superior to doxorubicin alone was doxorubicin followed 1 hour later by flavopiridol (P = 0.01 at day 38). All the other combinations were either equal to (D → F4, P = 0.19) or inferior to (D → F7, P = 0.08 and F → D7, P = 0.71) doxorubicin alone. There was no significant weight loss with the combination therapy. Interestingly, when comparing flavopiridol with the D → F1 combination, there appeared to be a trend toward lower tumor growth with the combination; however, this did not reach statistical significance (P = 0.15). These data were consistent with prior observations that dedifferentiated liposarcoma cell lines, with amplified CDK4, are highly sensitive to flavopiridol (data not shown) as well as the clinical observation that dedifferentiated liposarcomas are generally resistant to doxorubicin.

Given these findings, we launched a phase I dose-escalation trial of flavopiridol in combination with doxorubicin. The primary objective of this trial was to determine the MTD of flavopiridol in combination with doxorubicin in patients with advanced sarcomas. Secondary objectives were to investigate the clinical PK of flavopiridol in combination with doxorubicin and to obtain preliminary data on the therapeutic activity of this regimen.

From October 13, 2004, to January 14, 2010, 31 patients with metastatic or locally recurrent sarcoma were registered and 30 were treated. One patient was not evaluable for determining DLT as that patient did not initiate treatment after registration (withdrew consent). Three patients were not evaluable for determining response because they did not complete 2 cycles of treatment. The reasons were withdrawal of consent (2 patients) and clinical deterioration (1 patient).

Table 2 lists the patient characteristics of the 31 patients who were accrued to the protocol. The median age was 57 years (range, 31–72 years) and the median Karnofsky performance status was 90% (range, 70%–100%). There were 20 men and 11 women. The sarcomas treated and patient numbers were liposarcoma (16: 15 of which were WD/DD, 1 pleomorphic), leiomyosarcoma (n = 5), fibrosarcoma (n = 3), MPNST (n = 2), undifferentiated pleomorphic sarcoma (n = 1), osteosarcoma (extraosseous; n = 1), rhabdomyosarcoma (pleomorphic; n = 1), gastrointestinal stromal tumor (n = 1), and solitary fibrous tumor (n = 1). The majority of patients had not received prior chemotherapy (16% pretreated) with an overall range of 0 to 2 prior treatments. No patients had received prior doxorubicin and all had progressive disease at the time of enrollment with an indication to begin systemic treatment.

Table 3 lists the most common grade II to IV hematologic toxicities for the first cycle of therapy (no relevant nonhematologic toxicities were observed in cycle 1). Combination treatment with 60 mg/m² of doxorubicin and flavopiridol as a 1-hour bolus was documented to be well tolerated without DLT to the protocol-specified levels. Starting at 60 mg/m² of flavopiridol, grade III lymphopenia was also observed, a common non–dose-limiting hematologic toxicity of flavopiridol (21). A single cohort of patients was treated with 75 mg/m² of doxorubicin and 60 mg/m² of flavopiridol without DLT (cohort 5). However, there was grade III neutropenia in all 3 patients treated at this dose level, suggesting that the increased doxorubicin dose resulted in increased neutropenia.

Starting with 70 mg/m² of total flavopiridol in cohort 6, the combination of doxorubicin and split dosing flavopiridol was also found to be tolerable. However, DLTs were observed in the seventh and eighth cohorts. In the seventh cohort, one patient experienced DLT including both grade IV neutropenia and leukopenia leading to expansion of the cohort to 6 patients. In cohort 8, one patient experienced DLT including grade IV neutropenia and leukopenia and grade III febrile neutropenia. This cohort was also expanded without observance of further significant toxicity. Beyond these, no other DLTs were observed throughout the study. Dose-escalation beyond cohort 8 was not pursued, as the prespecified maximal flavopiridol dose of 90 mg/m² was achieved. As such, no MTD was formally established. However, it should be noted that there was substantial grade III neutropenia in cohort 7 (3 of 6 patients) and cohort 8 (5 of 6 patients) with 2 episodes of grade III thrombocytopenia. Therefore, it is unlikely that further dose-escalation could have been safely achieved. Notably, all DLTs were hematologic in nature and tumor lysis syndrome was not observed.

Supplementary Table S1 lists the most common grade II to IV cumulative hematologic toxicities for all cycles of treatment. The cumulative pattern of toxicity was similar to that in cycle 1 of treatment and was principally limited to hematologic effects. For the whole study population, the most common grade III/IV toxicities were neutropenia (45%), leukopenia (26%), thrombocytopenia (9%), and lymphopenia (6%). Grade III/IV neutropenia occurred in 14% of patients on the bolus schedule versus 33% of those on the bolus/infusional schedule. Similarly, grade III/IV leukopenia occurred in 4% and 23% for the bolus-only and split dosing schedules, respectively. Grade III/IV thrombocytopenia occurred in 0% and 9% of patients on the bolus-only and split dosing schedules, respectively. The rates of lymphopenia and anemia were not impressively different between the 2 schedules.

Several significant nonhematologic toxicities occurred after cycle 1, however, were generally not attributed to study treatment and were not considered dose-limiting for the protocol. The majority of these occurred with the split dosing flavopiridol schedules. The exception to this was the one death on protocol (cohort 3). This was due to a small bowel perforation and was attributed to treatment. The other toxicities (attributed as not related to treatment) included the development of grade IV central nervous system ischemia and a grade III pulmonary embolus in 2 different patients in dose cohort 6, a grade III pulmonary embolus with left ventricular dysfunction (following 2 cycles of therapy), a grade IV small bowel perforation in 2 separate patients in dose cohort 7, and both a grade III psychosis and grade IV confusion in a patient in dose cohort 8.

Blood samples from 31 patients were obtained to conduct PK analyses of flavopiridol. Table 4 summarizes maximum observed plasma concentration (C_max) across all subjects in a cohort. Flavopiridol C_max was observed at the first time point analyzed in each schedule (0.5 hours) and ranged from 2.22 μmol/L (SD, 0.07) at a flavopiridol dose of 50 mg/m², up to a maximum of 3.33 μmol/L (SD, 1.38) at a flavopiridol dose 90 (50/40) mg/m². Use of the split dosing schedule did not appear to increase the C_max of flavopiridol. There was significant interpatient variability within some cohorts (3, 7, and 8). The dose of doxorubicin was held constant for all but one dose level thus making evaluation of any potential interaction between the 2 drugs difficult. Notably however, the C_max range noted in this trial is similar to that seen in previous clinical trials combining flavopiridol with other chemotherapies (24, 25), suggesting that flavopiridol exposure is unlikely to be significantly altered when combined with doxorubicin. In cohorts 7 and 8, there was higher C_max in patients who experienced DLT as opposed to those who did not (data not shown), but this difference did not meet statistical significance. There were insufficient PK time points available to formally evaluate the area under the curve (AUC) for either schedule.

Twenty-eight patients were evaluable for response assessment (Table 5). There were 2 RECIST 1.0 partial responses and 14 patients had RECIST stable disease (SD) as best response (median, 15 weeks; range, 3–99 weeks). The disease control rate [complete response (CR) + PR + SD for >3 months) was 57% (16 of 28). There was no apparent difference by flavopiridol schedule. Of the 12 evaluable patients with WD and DD liposarcoma, 5 had progression of disease at first evaluation whereas 7 had stable disease for at least 3 months as best response (median, 20 weeks; range, 12–99 weeks). One notable patient (WD and DD liposarcoma) was maintained on study for 99 weeks. This patient, treated in cohort 1, initially received the maximum allowable doxorubicin dose (including use of dexrazoxane) and was thereafter continued on single-agent flavopiridol. The best response was stable disease, which was maintained through 83 weeks. At that point, the patient decided to withdraw consent for further treatment. Thirty-three weeks later, the patient was noted to have progression of disease and under a special protocol amendment was reinitiated on single-agent flavopiridol. Disease stability was recovered and the patient continued on treatment for further 16 weeks before eventually progressing.

We examined the use of flavopiridol in potentiating the effects of doxorubicin on the growth of malignant sarcoma both in vitro and in vivo. Preclinically, we documented in MPNST cells that flavopiridol potentiates doxorubicin, when compared with single agents alone. Furthermore, we showed that flavopiridol is active in vivo both as a single agent and in combination with doxorubicin in liposarcoma xenograft with amplified CDK4.

Given these findings, we conducted a phase I dose-escalation clinical trial of flavopiridol plus doxorubicin in patients with advanced sarcomas. Biologically active and therapeutic doses of flavopiridol (90 mg/m²; 50 mg/m² bolus followed by 40 mg/m² infusion) and doxorubicin (60 mg/m²) were combined without reaching an MTD. The achieved dose of flavopiridol was similar to that shown to be tolerable in combination with other chemotherapies and the PK at most of the dose levels tested were in the active range based on preclinical data (13, 26). Hematologic DLTs, constituted by neutropenia, leukopenia, lymphopenia, and thrombocytopenia, were observed by the combination of flavopiridol and anthracycline chemotherapy. Adverse events were generally tolerable, with the appearance of febrile neutropenia in only one instance. We conclude that flavopiridol can be combined with doxorubicin safely at biologically active doses.

On the basis of the results of the clinical study, it is not possible to make a definite determination whether the bolus schedule or the split dosing schedule is preferred for future clinical development of flavopiridol in combination with doxorubicin or more generally in the treatment of sarcoma. Regarding safety, no MTD was reached. Dose-limiting hematologic toxicity was increased with the split dosing regimen and this became more evident with cumulative dosing. Nonhematologic toxicity also became more apparent with cumulative dosing on the divided dose flavopiridol schedule. Unlike studies using a split-dose schedule for the treatment of hematologic malignancies, no evidence of tumor lysis syndrome was observed in this study.

In regard to efficacy, there were 2 partial responses, as well as stable disease as long as 99 weeks. Disease control (PR + SD > 3 months) was documented at various dose levels and was independent of dosing schedules of flavopiridol. Interpatient variability, especially in dose levels 3, 7, and 8, somewhat confounds the use of PK to determine the most efficacious dose and schedule. The flavopiridol C_max generally increased with increasing total flavopiridol dose with all dose cohorts having C_max greater than 2 μmol/L. Given this, it also does not appear that continuous flavopiridol exposure via infusion added significant clinical benefit. In view of the overall increased toxicity with the divided dose schedule, the bolus schedule would seem to be preferred for future development of flavopiridol in combination with doxorubicin for the ambulatory treatment of sarcoma.

The combination of flavopiridol and doxorubicin provided a substantial disease control in this study, with 68% (19 of 28) achieving PR or SD as best result. This is especially interesting given that 12 of the 28 evaluable patients had a diagnosis of WD and DD liposarcoma, a disease that is generally nonchemotherapy responsive but in which CDK4 is frequently amplified (27). Within this subpopulation, the disease control rate was 67%. For the entire study, the progression-free survival rate at 12 weeks (PFS_12weeks) was 57% (16 of 28) and progression-free survival rate at 24 weeks (PFS_24weeks) was 32% (9 of 28). Given the heterogeneity of soft tissue sarcomas, the European Organization for Research and Treatment of Cancer (EORTC) has developed standards for evaluating new treatments for soft tissue sarcoma in the phase II setting (28). These standards incorporate progression-free survival and clinical benefit in evaluation of new agents. Notably, in this study, the PFS_12weeks and PFS_24weeks compare favorably with these reference standards and suggest that this regimen may be worth further exploration in this patient population.

While the benefit of flavopiridol-based therapy in the treatment of WD and DD liposarcoma could be hypothesized to be a function of its CDK4 amplification, other sarcoma types are not as clearly linked to dysregulated apoptosis. In this study, we note prolonged SD in various tumor types such as leiomyosarcoma, fibrosarcoma, and pleomorphic rhabdomyosarcoma. While these tumors are also associated with chemotherapy responsiveness to anthracyclines, it is possible that doxorubicin was potentiated by flavopiridol. Recent literature has suggested that the predominant mechanism of flavopiridol efficacy is through inhibition of CDK9 (29). This results in suppression of critical antiapoptotic molecules and may lead to the potentiation of chemotherapy. Considering this, a rational mechanism for chemotherapy potentiation by flavopiridol would appear to be promotion of a death response in tumor cells after insult by chemotherapy. Given these observations, further study of flavopiridol in the treatment of WD and DD liposarcomas, and soft tissue sarcomas more generally, is warranted.

M.A. Dickson and S. Singer are consultants/advisory board members for Pfizer. No potential conflicts of interests were disclosed by other authors.

Conception and design: J.J. Luke, D.R. D'Adamo, M.A. Dickson, E. de Stanchina, S. Singer, G.K. Schwartz

Development of methodology: D.R. D'Adamo, M.A. Dickson, G.K. Schwartz

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): D.R. D'Adamo, M.A. Dickson, M.L. Keohan, R.D. Carvajal, R.G. Maki, E. de Stanchina, E. Musi, S. Singer, G.K. Schwartz

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): J.J. Luke, D.R. D'Adamo, M.A. Dickson, M.L. Keohan, R.D. Carvajal, E. de Stanchina, E. Musi, S. Singer, G.K. Schwartz

Writing, review, and/or revision of the manuscript: J.J. Luke, D.R. D'Adamo, M.A. Dickson, M.L. Keohan, R.D. Carvajal, R.G. Maki, S. Singer, G.K. Schwartz

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): J.J. Luke, D.R. D'Adamo, S. Singer, G.K. Schwartz

Study supervision: D.R. D'Adamo, S. Singer, G.K. Schwartz

The study was supported by Soft Tissue Sarcoma Program Project grant P01 CA 047179.

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.