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Mechanism of Action
Tretinoin is not a cytolytic agent but instead induces cytodifferentiation and decreased proliferation of APL cells in culture and in vivo. In APL patients, tretinoin treatment produces an initial maturation of the primitive promyelocytes derived from the leukemic clone, followed by a repopulation of the bone marrow and peripheral blood by normal, polyclonal hematopoietic cells in patients achieving complete remission (CR). The exact mechanism of action of tretinoin in APL is unknown.
Tretinoin activity is primarily due to the parent drug. In human pharmacokinetics studies, orally administered drug was well absorbed into the systemic circulation, with approximately two-thirds of the administered radiolabel recovered in the urine. The terminal elimination half-life of tretinoin following initial dosing is 0.5 to 2 hours in patients with APL. There is evidence that tretinoin induces its own metabolism. Plasma tretinoin concentrations decrease on average to one-third of their day 1 values during 1 week of continuous therapy. Mean ± SD peak tretinoin concentrations decreased from 394 89 to 138 ± 139 ng/mL, while area under the curve (AUC) values decreased from 537 ± 191 ng·h/mL to 249 ± 185 ng·h/mL during 45 mg/m2 daily dosing in 7 APL patients. Increasing the dose to "correct" for this change has not increased response.
A single 45 mg/m2 (~80 mg) oral dose to APL patients resulted in a mean SD peak tretinoin concentration of 347 ± 266 ng/mL. Time to reach peak concentration was between 1 and 2 hours.
The apparent volume of distribution of tretinoin has not been determined. Tretinoin is greater than 95% bound in plasma, predominately to albumin. Plasma protein binding remains constant over the concentration range of 10 to 500 ng/mL.
Tretinoin metabolites have been identified in plasma and urine. Cytochrome P450 enzymes have been implicated in the oxidative metabolism of tretinoin. Metabolites include 13-cis retinoic acid, 4-oxo trans retinoic acid, 4-oxo cis retinoic acid, and 4-oxo trans retinoic acid glucuronide. In APL patients, daily administration of a 45 mg/m2 dose of tretinoin resulted in an approximately tenfold increase in the urinary excretion of 4-oxo trans retinoic acid glucuronide after 2 to 6 weeks of continuous dosing, when compared to baseline values.
Studies with radiolabeled drug have demonstrated that after the oral administration of 2.75 and 50 mg doses of tretinoin, greater than 90% of the radioactivity was recovered in the urine and feces. Based upon data from 3 subjects, approximately 63% of radioactivity was recovered in the urine within 72 hours and 31% appeared in the feces within 6 days.
The pharmacokinetics of tretinoin have not been separately evaluated in women, in members of different ethnic groups, or in individuals with renal or hepatic insufficiency.
In 13 patients who had received daily doses of tretinoin for 4 consecutive weeks, administration of ketoconazole (400 to 1200 mg oral dose) 1 hour prior to the administration of the tretinoin dose on day 29 led to a 72% increase (218 224 vs 375 ± 285 ng·h/mL) in tretinoin mean plasma AUC. The precise cytochrome P450 enzymes involved in these interactions have not been specified; CYP 3A4, 2C8 and 2E have been implicated in various preliminary reports.
VESANOID (tretinoin) has been investigated in 114 previously treated APL patients and in 67 previously untreated ("de novo") patients in one open-label, uncontrolled single investigator clinical study (Memorial Sloan-Kettering Cancer Center [MSKCC]) and in two cohorts of compassionate cases treated by multiple investigators under the auspices of the National Cancer Institute (NCI). All patients received 45 mg/m2/day as a divided oral dose for up to 90 days or 30 days beyond the day that CR was reached. Results are shown in the following table:
|MSKCC||NCI Cohort 1||NCI Cohort 2|
|Complete Remission||16 (80%)||11 (73%)||24 (50%)||5 (36%)||24 (52%)||26 (68%)|
|Median Survival (Mo)||10.8||NR||5.8||0.5||8.8||NR|
|Median Follow-up (Mo)||9.9||42.9||5.6||1.2||8.0||13.1|
|RA-APL Syndrome||4 (20%)||5 (33%)||10 (21%)||6 (43%)||NA||NA|
|NR = Not Reached
NA = Not Available
*Including 9 chemorefractory patients
†Including 8 patients who received chemotherapy but failed to enter remission
The median time to CR was between 40 and 50 days (range: 2 to 120 days). Most patients in these studies received cytotoxic chemotherapy during the remission phase. These results compare to the 30% to 50% CR rate and ≤ 6 month median survival reported for cytotoxic chemotherapy of APL in the treatment of relapse.
Ten of 15 pediatric cases achieved CR (8 of 10 males and 2 of 5 females). There were insufficient patients of black, Hispanic or Asian derivation to estimate relative response rates in these groups, but responses were seen in each category.
Responses were seen in 3 of 4 patients for whom cytogenetic analysis failed to detect the t(15;17) translocation typically seen in APL. The t(15;17) translocation results in the PML/RARα gene, which appears necessary for this disease. Molecular genetic studies were not conducted in these cases, but it is likely they represent cases with a masked translocation giving rise to PML/RARα. Responses to tretinoin have not been observed in cases in which PML/RARα fusion has been shown to be absent.
Last reviewed on RxList: 9/24/2008
This monograph has been modified to include the generic and brand name in many instances.
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