Mechanism of Action
Ixabepilone is a semi-synthetic analog of epothilone B. Ixabepilone binds directly
to β-tubulin subunits on microtubules, leading to suppression of microtubule
dynamics. Ixabepilone suppresses the dynamic instability of αβ-II
and αβ-III microtubules. Ixabepilone possesses low in vitro
susceptibility to multiple tumor resistance mechanisms including efflux transporters,
such as MRP-1 and P-glycoprotein (P-gp). Ixabepilone blocks cells in the mitotic
phase of the cell division cycle, leading to cell death.
Pharmacodynamics
In cancer patients, ixabepilone has a plasma concentration-dependent effect
on tubulin dynamics in peripheral blood mononuclear cells that is observed as
the formation of microtubule bundles. Ixabepilone has antitumor activity in
vivo against multiple human tumor xenografts, including drug-resistant types
that overexpress P-gp, MRP-1, and βIII tubulin isoforms, or harbor tubulin
mutations. Ixabepilone is active in xenografts that are resistant to multiple
agents including taxanes, anthracyclines, and vinca alkaloids. Ixabepilone demonstrated
synergistic antitumor activity in combination with capecitabine in vivo.
In addition to direct antitumor activity, ixabepilone has antiangiogenic activity.
Pharmacokinetics
Absorption
Following administration of a single 40 mg/m2 dose of IXEMPRA in
patients with cancer, the mean Cmax was 252 ng/mL (coefficient of variation,
CV 56%) and the mean AUC was 2143 ng•hr/mL (CV 48%). Typically Cmax occurred
at the end of the 3 hour infusion. In cancer patients, the pharmacokinetics
of ixabepilone were linear at doses of 15 to 57 mg/m2.
Distribution
The mean volume of distribution of 40 mg/m2 ixabepilone at steady-state
was in excess of 1000 L. In vitro, the binding of ixabepilone to human serum proteins ranged from 67 to 77%, and the blood-to-plasma concentration
ratios in human blood ranged from 0.65 to 0.85 over a concentration range of
50 to 5000 ng/mL.
Metabolism
Ixabepilone is extensively metabolized in the liver. In vitro studies
indicated that the main route of oxidative metabolism of ixabepilone is via
CYP3A4. More than 30 metabolites of ixabepilone are excreted into human urine
and feces. No single metabolite accounted for more than 6% of the administered
dose. The biotransformation products generated from ixabepilone by human liver
microsomes were not active when tested for in vitro cytotoxicity against
a human tumor cell line.
In vitro studies using human liver microsomes indicate that clinically
relevant concentrations of ixabepilone do not inhibit CYP3A4, CYP1A2, CYP2A6,
CYP2B6, CYP2C8, CYP2C9, CYP2C19, or CYP2D6. Ixabepilone does not induce the
activity or the corresponding mRNA levels of CYP1A2, CYP2B6, CYP2C9, or CYP3A4
in cultured human hepatocytes at clinically relevant concentrations. Therefore,
it is unlikely that ixabepilone will affect the plasma levels of drugs that
are substrates of CYP enzymes.
Elimination
Ixabepilone is eliminated primarily as metabolized drug. After an intravenous
14[C]-ixabepilone dose to patients, approximately 86% of the dose
was eliminated within 7 days in feces (65% of the dose) and in urine (21% of
the dose). Unchanged ixabepilone accounted for approximately 1.6% and 5.6% of
the dose in feces and urine, respectively. Ixabepilone has a terminal elimination
half-life of approximately 52 hours. No accumulation in plasma is expected for
ixabepilone administered every 3 weeks.
Effects of Age, Gender, and Race
Based upon a population pharmacokinetic analysis in 676 cancer patients, gender, race, and age do not have meaningful effects on the pharmacokinetics of ixabepilone.
Animal Toxicology
Overdose
In rats, single intravenous doses of ixabepilone from 60 to 180 mg/m2 (mean
AUC values ≥ 8156 ng•h/mL) were associated with mortality occurring between
5 and 14 days after dosing, and toxicity was principally manifested in
the gastrointestinal, hematopoietic (bone-marrow), lymphatic, peripheral-nervous,
and male-reproductive systems. In dogs, a single intravenous dose of 100 mg/m2
(mean AUC value of 6925 ng•h/mL) was markedly toxic, inducing severe gastrointestinal
toxicity and death 3 days after dosing.
Clinical Studies
Combination Therapy
In an open-label, multicenter, multinational, randomized trial of 752 patients
with metastatic or locally advanced breast cancer, the efficacy and safety of
IXEMPRA (40 mg/m2 every 3 weeks) in combination with capecitabine
(at 1000 mg/m2 twice daily for 2 weeks followed by 1 week rest) were
assessed in comparison with capecitabine as monotherapy (at 1250 mg/m2
twice daily for 2 weeks followed by 1 week rest). Patients were previously treated
with anthracyclines and taxanes. Patients were required to have demonstrated
tumor progression or resistance to taxanes and anthracyclines as follows:
- tumor progression within 3 months of the last anthracycline dose in the
metastatic setting or recurrence within 6 months in the adjuvant or neoadjuvant
setting, and
- tumor progression within 4 months of the last taxane dose in the metastatic
setting or recurrence within 12 months in the adjuvant or neoadjuvant setting.
For anthracyclines, patients who received a minimum cumulative dose of 240
mg/m2 of doxorubicin or 360 mg/m2 of epirubicin were also
eligible.
Sixty-seven percent of patients were White, 23% were Asian, and 3% were Black. Both arms were evenly matched with regards to race, age (median 53 years), baseline performance status (Karnofsky 70-100%), and receipt of prior adjuvant or neo-adjuvant chemotherapy (75%). Tumors were ER-positive in 47% of patients, ER-negative in 43%, HER2-positive in 15%, HER2-negative in 61%, and ER-negative, PR-negative, HER2-negative in 25%. The baseline disease characteristics and previous therapies for all patients (n=752) are shown in Table 6.
Table 6: Baseline Disease Characteristics and Previous Therapies
| |
IXEMPRA with capecitabine
n=375 |
Capecitabine
n=377 |
| Site of disease |
| Visceral disease (liver or lung) |
316 (84%) |
315 (84%) |
| Liver |
245 (65%) |
228 (61%) |
| Lung |
180 (48%) |
174 (46%) |
| Lymph node |
250 (67%) |
249 (66%) |
| Bone |
168 (45%) |
162 (43%) |
| Skin/soft tissue |
60 (16%) |
62 (16%) |
| Number of prior chemotherapy regimens in metastatic settinga
|
| 0 |
27 (7%) |
33 (9%) |
| 1 |
179 (48%) |
184 (49%) |
| 2 |
152 (41%) |
138 (37%) |
| ≥ 3 |
17 (5%) |
22 (6%) |
| Anthracycline resistanceb |
164 (44%) |
165 (44%) |
| Taxane Resistancec |
| Neoadjuvant/adjuvant setting |
40 (11%) |
44 (12%) |
| Metastatic setting |
327 (87%) |
319 (85%) |
a For IXEMPRA plus capecitabine versus capecitabine
only, prior treatment in the metastatic setting included cyclophosphamide
(25% vs. 23%), fluorouracil (22% vs. 16%), vinorelbine (11% vs. 12%), gemcitabine
(9% each arm), carboplatin (9% vs. 7%), liposomal doxorubicin (3% each arm),
and cisplatin (2% vs. 3%).
b Tumor progression within 3 months in the metastatic setting
or recurrence within 6 months in the adjuvant or neoadjuvant setting.
c 24% and 21% of patients had received 2 or more taxane-containing
regimens in the combination and single agent treatment groups, respectively.
|
The patients in the combination treatment group received a median of 5 cycles of treatment and patients in the capecitabine monotherapy treatment group received a median of 4 cycles of treatment.
The primary endpoint of the study was progression-free survival (PFS) defined as time from randomization to radiologic progression as determined by Independent Radiologic Review (IRR), clinical progression of measurable skin lesions or death from any cause. Other study endpoints included objective tumor response based on Response Evaluation Criteria in Solid Tumors (RECIST), time to response, response duration, and overall survival.
IXEMPRA in combination with capecitabine resulted in a statistically significant improvement in PFS compared to capecitabine. The results of the study are presented in Table 7 and Figure 1.
Table 7: Efficacy of IXEMPRA in Combination with Capecitabine
vs Capecitabine Alone – Intent-to-Treat Analysis
| Efficacy Parameter |
IXEMPRA with Capecitabine
n=375 |
Capecitabine
n=377 |
| PFS |
| Number of eventsa |
242 |
256 |
| Median |
5.7 months |
4.1 months |
| (95% CI) |
(4.8 - 6.7) |
(3.1 - 4.3) |
| Hazard Ratiob (95% CI) |
0.69 (0.58 - 0.83) |
| p-valuec (Log rank) |
< 0.0001 |
| Objective Tumor Response Rate (95% CI) |
34.7% (29.9 - 39.7) |
14.3% (10.9 - 18.3) |
| p-valuec (CMH)d |
< 0.0001 |
| Duration of Response, Median (95% CI) |
6.4 months (5.6 - 7.1) |
5.6 months (4.2 - 7.5) |
a Patients were censored for PFS at the last date
of tumor assessment prior to the start of subsequent therapy. In patients
where independent review was not available PFS was censored at the randomization
date.
b For the hazard ratio, a value less than 1.00 favors combination
treatment.
c Stratified by visceral metastasis in liver/lung, prior chemotherapy
in metastatic setting, and anthracycline resistance.
d Cochran-Mantel-Haenszel test |
Figure 1: Progression-free Survival Kaplan Meier Curves
There was no statistically significant difference in overall survival between treatment arms in this study, as well as in a second similar study. In the study described above, the median overall survivals were 12.9 months (95% CI: 11.5, 14.2) in the combination therapy arm and 11.1 months (95% CI: 10.0, 12.5) in the capecitabine alone arm [Hazard Ratio 0.90 (95% CI: 0.77, 1.05), p-value=0.19].
In the second trial, comparing IXEMPRA in combination with capecitabine versus capecitabine alone, conducted in 1221 patients pretreated with an anthracycline and taxane, the median overall survivals were 16.4 months (95% CI: 15.0, 17.9) in the combination therapy arm and 15.6 months (95% CI: 13.9, 17.0), in the capecitabine alone arm [Hazard Ratio 0.90 (95% CI: 0.78, 1.03), p-value=0.12].
Monotherapy
IXEMPRA was evaluated as a single agent in a multicenter single-arm study in
126 women with metastatic or locally advanced breast cancer. The study enrolled
patients whose tumors had recurred or had progressed following two or more chemotherapy
regimens including an anthracycline, a taxane, and capecitabine. Patients who
had received a minimum cumulative dose of 240 mg/m2 of doxorubicin
or 360 mg/m2 of epirubicin were also eligible. Tumor progression
or recurrence were prospectively defined as follows:
- Disease progression while on therapy in the metastatic setting (defined
as progression while on treatment or within 8 weeks of last dose),
- Recurrence within 6 months of the last dose in the adjuvant or neoadjuvant
setting (only for anthracycline and taxane),
- HER2-positive patients must also have progressed during or after discontinuation
of trastuzumab.
In this study, the median age was 51 years (range, 30-78), and 79% were White, 5% Black, and 2% Asian, Karnofsky performance status was 70-100%, 88% had received two or more prior chemotherapy regimens for metastatic disease, and 86% had liver and/or lung metastases. Tumors were ER-positive in 48% of patients, ER-negative in 44%, HER2-positive in 7%, HER2-negative in 72%, and ER-negative, PR-negative, HER2-negative in 33%.
IXEMPRA was administered at a dose of 40 mg/m2 intravenously over
3 hours every 3 weeks. Patients received a median of 4 cycles (range 1 to 18)
of IXEMPRA therapy.
Objective tumor response was determined by independent radiologic and investigator review using RECIST. Efficacy results are presented in Table 8.
Table 8: Efficacy of IXEMPRA in Metastatic and Locally Advanced
Breast Cancer
| Endpoint |
Result |
| Objective tumor response rate (95% CI) |
| IRR Assessmenta (n=113) |
12.4% (6.9 - 19.9) |
| Investigator Assessment (n=126) |
18.3% (11.9 - 26.1) |
| Time to responseb (n=14) Median, weeks (min - max) |
6.1 (5 - 54.4) |
| Duration of responseb (n=14) Median, months (95% CI) |
6.0 (5.0 - 7.6) |
a All responses were partial.
b As assessed by IRR. |
Last updated on RxList: 10/15/2009