Modeling and simulation of neutropenia to support dosing regimens for liposome-encapsulated paclitaxel easy-to-use (LEP- ETU)

13011 Background: LEP-ETU is a liposomal formulation of paclitaxel designed to reduce toxicities while maintaining or enhancing the efficacy of the drug. A PKPD model was used to characterize the time course of neutrophils in relation to blood exposure of LEP-ETU in patients with solid tumors. Simulations were performed to provide justification for dosing schedule in Phase 3 trials. Methods: Data from 63 patients in two phase I studies with advanced malignancies were utilized. LEP-ETU was infused intravenously over 90 min. at 135, 175, 225, 275, 325, and 375 mg/m2 every 21 days (Q3W). A separate cohort received 175 mg/m2 of LEP-ETU infused over 3 hours following a Q3W regimen. Population PKPD modeling and simulations were performed to assess the influence of dose and schedule (275 - 325 mg/m2 Q3W vs. 90 - 110 mg/m2 QW) on neutropenia. Results: The temporal relationship between circulating neutrophils and paclitaxel concentrations was adequately characterized using a model representative of progenitor cells undergoing differentiation and maturation into neutrophils. Mean predicted (%SEM) baseline ANC was 4.3 (6) x 109/L, representative of a normal neutrophil count. Mean transit time (%SEM) was 80 (8) hr, representing a post-mitotic time delay to neutrophil maturation. The neutrophil T1/2 was 14 hr, comparable to published values. The drug effect (Slope·Cp) on the proliferation rate constant is decreased by 9- and 91-fold when 10 or 100 units of drug is present, indicative of paclitaxel’s inhibition of progenitor cell production in bone marrow. Simulations suggest a higher risk of neutropenia with the Q3W regimen and dose-dependent severity for both the Q3W and QW regimens. The predicted incidence of grade 4 toxicity increased from 33% - 42% across the dose range of 275 - 325 mg/m2 Q3W. For a dose of 110 mg/m2 given QW, the incidence of grade 4 neutropenia was 16% compared to 42% for the same total dose of 325 mg/m2 given Q3W. Conclusions: Modeling and simulation predict that a Q3W regimen of 325 mg/m2 LEP- ETU (MTD) provides acceptable neutropenia outcomes comparable to 175 mg/m2 paclitaxel Q3W, while minimizing the Cremophor- associated adverse events. Future consideration of QW regimens may be explored. No significant financial relationships to disclose.

Timing of the First Cleavage Division of the Mouse and the Duration of its Component Stages: A Study of Living and Fixed Eggs

Fertilized mouse eggs were examined between 27 and 34 h after the superovulating injection of human chorionic gonadotrophin (HCG). Out of 1334 eggs examined, 432 were already at the 2-cell stage; the remaining 902 at the 1-cell stage were examined in detail. All chromosome preparations of the first cleavage mitosis were classified into groups corresponding with the stages of prometaphase (early and late), metaphase (early or ‘prechromatid’, ‘chromatid’ and ‘late chromatid’) and anaphase. An indirect estimate was made of the duration of the first cleavage mitosis and of its component stages from the incidence of stages observed at different time intervals after the HCG injection. Fertilized eggs were also observed at 37°C by time-lapse cine-photography and the interval between the disappearance of the pronuclei and the beginning of telophase of the first cleavage division was determined. The progress of eggs fertilized in vitro was also observed under normal culture conditions. A close correlation was observed between the indirect method of assessing the mitotic time and the direct values obtained from the studies on time-lapse and in vitro culture. The effect of temperature on the mitotic time was also examined by the time-lapse method.

Variability of Proliferative Patterns in Acute Lymphoid Leukemia of Children

Proliferative patterns of neoplastic bone marrow in five children with untreated acute lymphoid leukemia (ALL) were studied radioautographically using a combined analysis of labeling indices and median grain counts of blast cells in mitosis and in interphase as a function of time after a single intravenous injection of 3H-thymidine. The results are interpreted as follows: (1) In at least two cases, a considerable fraction of initially labeled large blasts immediately reentered another cell cycle on completion of mitosis, as evidenced from periodicities of mitotic labeling curves. The relative number of immediately recycling cells varied from patient to patient. In all cases a certain percentage of cells following division passed through a nonproliferative phase of variable duration characterized by a small nuclear size. (2) The time parameters of the mitotic cycle of leukemic blasts differed from patient to patient, although the generation times of immediately recycling cells were close to 50 hr (47-55 hr) in at least three cases. The duration of S phase varied between approximately 20 and 40 hr. Best estimates for the duration of G2 + ½ M ranged from 3 to 7 hr, and for mitotic time from 0.5 to 1.6 hr. (3) Leukemic bone marrow cells in all five cases of ALL had longer generation times than any nonneoplastic lymphopoietic or hemopoietic cell line of man investigated so far. (4) A continuous reentry of small leukemic blast cells into a proliferative phase and a concomitant increase in nuclear size occurred in all cases studied, thus confirming previous results obtained by other authors. Possible reutilization of label and its implications with regard to interpreting cytokinetic data in ALL are discussed.

The Chronology of the Mitotic Cycle of Human Granulocytopoietic Cells Phase Contrast Studies on Living Cells in Vitro

Data on the mitotic index of human granulocytopoietic cells are presented. From these and from the duration of mitosis directly measured in living cells by phase contrast microscope, the weighted average generation time and the mean compartment transit time are computed. Maturation in granulocytopoietic cells appears to induce a reduction of mitotic indices and mitotic rate and an increase in mitotic time and in mean compartment transit time. Part of the increment in mitotic duration may be due to the acquisition by a part of the granulocytopoietic cells of cytoplasmic peripheral motility or other specialized activities, thus distracting part of the energies destined to mitosis.

Granulocytopoiesis in Human Bone Marrow Cultures Studied by Means of Kinematography

1. In short-term human bone marrow cultures, the generation times of three myelocytes were measured by phase contrast kinematography to be 29-30.5 hours. Two of these myelocyte generations were in direct succession of each other. 2. The mean mitotic time of 93 myelocytic mitoses was found to be 88 ± 20 minutes normally and 142 ± 13 minutes in 13 leukemic blast cells. This difference was statistically significant. 3. The maturation of myelocytic cells after their last mitosis from the metamyelocyte into mature granulocytes was followed and found to vary between 7 and 37 hours. This time difference was the result of the fact that one of the cells underwent a further division, where the three sister cells did not divide but matured only. All five final cells acquired the characteristics of band forms at about the same time. In eight other cells, the metamyelocyte-bandform transition time was found to be 7 to 30 hours. In all cells studied, the morphologic characteristics changed repeatedly between those considered typical for myelocytes and metamyelocytes. Thus, metamyelocytes were found to undergo division under the conditions of this study. 4. No evidence was found for amitotic divisions. However, the development of tetraploid cells was observed to be the result of endomitosis, of postmitotic formation of binucleated cells, or of formation of giant metamyelocytes by nuclear fusion of a binucleated cell.