The in vitro cytotoxicity observed for etoposide phosphate is significantly
less than that seen with etoposide which is believed due to the necessity for
conversion in vivo to the active moiety, etoposide, by dephosphorylation.
The mechanism of action is believed to be the same as that of etoposide. Etoposide
has been shown to cause metaphase arrest in chick fibroblasts. Its main effect,
however, appears to be at the G2 portion of the cell cycle in mammalian
cells. Two different dose-dependent responses are seen. At high concentrations
(10 μg/mL or more), lysis of cells entering mitosis is observed. At low concentrations
(0.3 to 10 μg/mL), cells are inhibited from entering prophase. It does not
interfere with microtubular assembly. The predominant macromolecular effect
of etoposide appears to be the induction of DNA strand breaks by an interaction
with DNA-topoisomerase II or the formation of free radicals.
ETOPOPHOS Bioequivalence
Following intravenous administration of ETOPOPHOS, etoposide phosphate is rapidly
and completely converted to etoposide in plasma. A direct comparison of the
pharmacokinetic parameters [area under the concentration time curve (AUC) and
the maximum plasma concentration (CMAX)] of etoposide following intravenous
administration of molar equivalent doses of ETOPOPHOS and VePesid was made in
two randomized crossover studies in patients with a variety of malignancies.
In the first study of 41 evaluable patients, the etoposide mean ± S.D.
AUC values were 168.3 ± 48.2 μg•hr/mL and 156.7 ± 43.4
μg hr/mL following administration of molar equivalent doses of 150 mg/m²
ETOPOPHOS or VePesid with a 3.5-hour infusion time; the corresponding mean ±
S.D. CMAX values were 20.0 ± 3.7 μg/mL and 19.6 ± 4.2 μg/mL,
respectively. The point estimate (90% confidence interval) for the bioavailability
of etoposide from ETOPOPHOS, relative to VePesid, was 107% (105%, 110%) for
AUC and 103% (99%, 106%) for CMAX. In the second study of 29 evaluable patients
following intravenous administration of 90, 100, and 110 mg/m² molar equivalents
of ETOPOPHOS or VePesid with a 60-minute infusion time, the etoposide mean ±
S.D. AUC values (normalized to the 100 mg/m² dose) were 96.1 ± 22.6
μg •hr/mL and 86.5 ± 25.8 μg •hr/mL, respectively;
the corresponding mean ± S.D. CMAX values (normalized to the 100 mg/m²
dose) were 20.1 ± 4.1 μg/mL and 19.0 ± 5.1 μg/mL, respectively.
The point estimate (90% confidence interval) for the bioavailability of etoposide
from ETOPOPHOS, relative to VePesid, was 113% (107%, 119%) for AUC and 107%
(101%, 113%) for CMAX indicating bioequivalence. Results from both studies demonstrated
no statistically significant differences in the AUC and CMAX parameters for
etoposide when administered as ETOPOPHOS or VePesid. In addition, in the latter
study, there were no statistically significant differences in the pharmacodynamic
parameters (hematologic toxicity) after administration of ETOPOPHOS or VePesid.
Following VePesid administration, the mean nadir values (expressed as percent
decrease from baseline) for leukocytes, granulocytes, hemoglobin, and thrombocytes
were 67.2 ± 17.0%, 84.1 ± 14.6%, 22.6 ± 9.8%, and 46.4
± 21.9%, respectively; the corresponding values after administration
of ETOPOPHOS were 67.3 ± 14.2%, 81.0 ± 16.5%, 21.4 ± 9.9%,
and 44.1 ± 20.7%, respectively.
Because of the similarity of pharmacokinetics and pharmacodynamics of etoposide
after administration of either ETOPOPHOS or VePesid, the following information
on VePesid should be considered:
VePesid Pharmacokinetics
On intravenous administration, the disposition of etoposide is best described
as a biphasic process with a distribution half-life of about 1.5 hours and terminal
elimination half-life ranging from 4 to 11 hours. Total body clearance values
range from 33 to 48 mL/min or 16 to 36 mL/min/m² and, like the terminal
elimination half-life, are independent of dose over a range 100-600 mg/m².
Over the same dose range, the AUC and the CMAX values increase linearly with
dose. Etoposide does not accumulate in the plasma following daily administration
of 100 mg/m² for 4 to 5 days. After intravenous infusion the CMAX and AUC
values exhibit marked intra- and inter-subject variability.
The mean volumes of distribution at steady state fall in the range of 18 to
29 liters or 7 to 17 L/m². Etoposide enters the CSF poorly. Although it
is detectable in CSF and intracerebral tumors, the concentrations are lower
than in extracerebral tumors and in plasma. Etoposide concentrations are higher
in normal lung than in lung metastases and are similar in primary tumors and
normal tissues of the myometrium. In vitro, etoposide is highly protein
bound (97%) to human plasma proteins. An inverse relationship between plasma
albumin levels and etoposide renal clearance is found in children. In a study
determining the effect of other therapeutic agents on the in vitro binding
of carbon-14 labeled etoposide to human serum proteins, only phenylbutazone,
sodium salicylate, and aspirin displaced protein-bound etoposide at concentrations
achieved in vivo.
Etoposide binding ratio correlates directly with serum albumin in patients
with cancer and in normal volunteers. The unbound fraction of etoposide significantly
correlated with bilirubin in a population of cancer patients. Data have suggested
a significant inverse correlation between serum albumin concentration and free
fraction of etoposide (see PRECAUTIONS).
After intravenous administration of 14C-etoposide (100-124 mg/m²),
mean recovery of radioactivity in the urine was 56% of the dose at 120 hours,
45% of which was excreted as etoposide; fecal recovery of radioactivity was
44% of the dose at 120 hours.
In children, approximately 55% of the dose of VePesid is excreted in the urine
as etoposide in 24 hours. The mean renal clearance of etoposide is 7 to 10 mL/min/m²
or 35% of the total body clearance over a dose of 80 to 600 mg/m². Etoposide,
therefore, is cleared by both renal and nonrenal processes, i.e., metabolism
and biliary excretion. The effect of renal disease on plasma etoposide clearance
is not known in children.
Biliary excretion of unchanged drug and/or metabolites is an important route
of etoposide elimination as fecal recovery of radioactivity is 44% of the intravenous
dose. The hydroxy acid metabolite [4'-demethylepipodophyllic acid-9-(4,6-O-(R)-ethylidene-β-D-gluco-pyranoside)],
formed by opening of the lactone ring, is found in the urine of adults and children.
It is also present in human plasma, presumably as the trans isomer. Glucuronide
and/or sulfate conjugates of etoposide are also excreted in human urine. Only
8% or less of an intravenous dose is excreted in the urine as radiolabeled metabolites
of 14C-etoposide. In addition, O-demethylation of the dimethoxyphenol
ring occurs through the CYP450 3A4 isoenzyme pathway to produce the corresponding
catechol.
In adults, the total body clearance of etoposide is correlated with creatinine
clearance, serum albumin concentration, and nonrenal clearance. Patients with
impaired renal function receiving etoposide have exhibited reduced total body
clearance, increased AUC, and a lower volume of distribution at steady state
(see PRECAUTIONS). Use of cisplatin therapy is associated with reduced
total body clearance. In children, elevated serum SGPT levels are associated
with reduced drug total body clearance. Prior use of cisplatin may also result
in a decrease of etoposide total body clearance in children.
Although some minor differences in pharmacokinetic parameters between age and
gender have been observed, these differences were not considered clinically
significant.
Clinical Studies
A total of seven clinical trials with 365 patients treated (368 entered) provide
the data base for the human experience summarized in this insert. Five phase
I trials evaluated etoposide phosphate given on a days 1, 3, and 5 or days 1
through 5 schedule. In two trials the drug was given over 5 minutes and in three
over 30 minutes. The following table summarizes the doses, schedules, infusion
times, and numbers of patients entered in the phase I experience.
Dose Escalation (Phase I) Trials of Etoposide Phosphate
| Study |
Schedule
Q 21 days |
Infusion
Time |
Dose Range
(mg/m²) |
Number of Patients
Entered |
| 002 |
Days 1-5 |
30 minutes |
25 - 110 |
68 |
| 005 |
Days 1,3,5 |
30 minutes |
50 - 175 |
39 |
| 006 |
Days 1-5 |
30 minutes |
50 - 125 |
28 |
| 008 |
Days 1,3,5 |
5 minutes |
50 - 200 |
36 |
| 009 |
Days 1-5 |
5 minutes |
50 - 125 |
27 |
Two trials evaluated the pharmacokinetic equivalence of etoposide and etoposide
phosphate. A phase I study (002) was expanded at the higher doses to compare
the pharmacokinetic profile of etoposide following administration of etoposide
or etoposide phosphate. Another multi-institutional trial (012) was conducted
at a dose of 150 mg/m² using a day 1, 3, and 5 schedule and a crossover
design.
The seventh trial (011) was a randomized study in which patients with limited
or extensive small cell lung cancer and no prior therapy were treated with either
cisplatin plus etoposide or cisplatin plus etoposide phosphate. Patients received
20 mg/m²/day of cisplatin for 5 days and 80 mg/m²/day of etoposide
or etoposide phosphate. A total of 121 patients were randomized and 120 treated
(60 per group). Response rates, time to response, duration of response, time
to progression, time to worsening performance status, and survival were similar
in the two groups whether the analysis was done for patients with limited or
extensive disease or for the entire population. The following table summarizes
the results regardless of disease extent.
Response to Treatment for All Patients
| |
Etoposide Phosphate plus Cisplatin |
Etoposide plus Cisplatin |
P-value |
| Complete Responses: |
15% |
15% |
1.000* |
| Partial Responses: |
46% |
43% |
0.855* |
| Overall Response Rate: |
61% |
58% |
0.854* |
| Median Time to Response: |
48 days |
46 days |
0.596** |
| Median Response Duration: |
273 days |
241 days |
0.141*** |
| Median Time to Progression: |
211 days |
213 days |
0.500*** |
| Median Time to Worsening Performance Status: |
210 days |
149 days |
0.472*** |
| Median Survival: |
348 days |
318 days |
0.780*** |
*Fisher's Exact test
**Wilcoxon Rank Sum test
***Logrank test |
The most prominent side effects were myelosuppression and GI toxicity. Sixty-eight
percent of patients treated with etoposide phosphate plus cisplatin had neutrophils
less than 500/mm³ at some time during treatment as did 88% of those getting
etoposide and cisplatin. Over 85% in each group had nausea and/or vomiting.
No differences in the pattern or severity of side effects were observed.
Last updated on RxList: 11/11/2008