A physiological model of granulopoiesis to predict clinical drug induced neutropenia from in vitro bone marrow studies: with application to a cell cycle inhibitor

Abstract

Neutropenia is one of the most common dose-limiting toxocities associated with anticancer drug therapy. The ability to predict the probability and severity of neutropenia based on in vitro studies of drugs in early drug development will aid in advancing safe and efficacious compounds to human testing. Toward this end, a physiological model of granulopoiesis and its regulation is presented that includes the bone marrow progenitor cell cycle, allowing for a mechanistic representation of the action of relevant anticancer drugs based on in vitro studies. Model development used data from previously reported tracer kinetic studies of granulocyte disposition in healthy humans to characterize the dynamics of neutrophil margination in the presence of endogenous granulocyte-colony stimulating factor (G-CSF). In addition, previously published data from healthy volunteers following pegfilgrastim and filgrastim were used to quantify the regulatory effects of support G-CSF therapies on granulopoiesis. The model was evaluated for the cell cycle inhibitor palbociclib, using an in vitro system of human bone marrow mononuclear cells to quantify the action of palbociclib on proliferating progenitor cells, including its inhibitory effect on G1 to S phase transition. The in vitro results were incorporated into the physiological model of granulopoiesis and used to predict the time course of absolute neutrophil count (ANC) and the incidence of neutropenia observed in three previously reported clinical trials of palbociclib. The model was able to predict grade 3 and 4 neutropenia due to palbociclib treatment with 86% accuracy based on in vitro data.

Keywords: Absolute neutrophil count; Cell cycle inhibitor; Granulocyte-colony stimulating factor; In vitro bone marrow toxicity; Neutrophil margination; Palbociclib.

Fig. 1:

The schematic diagram of the physiological neutropenia model. The dashed boxes represent the three major subsystems of the model: bone marrow (Eqs. (1)–(4), (9)), blood (Eqs. (5), (10)), and G-CSF (Eqs. (6)–(8)). The dotted lines indicate G-CSF’s action on proliferation, maturation, mobilization, and margination.

Fig. 2:

Mechanism of action of palbociclib incorporated in the cell cycle model. Minus sign represents palbociclib inhibition of k₁, plus sign represents palbociclib induction of the inactivation process.

Fig. 3:

Tracer kinetic margination model fit (average of four subjects – line) to the labeled neutrophil measurements (mean and standard deviation – symbols and error bars).

Fig. 4:

Model predicted (lines) and observed (symbols) pegfilgrastim serum concentration (upper panel left) and ANC (upper panel right) following the SC injection of 30, 60, 100, and 300 μg/kg pegfilgrastim. Lower panels show model predicted (lines) and observed (symbols) band cells (solid lines) and segmented neutrophils (dashed lines) for each of the four doses as indicated. Overall observed versus predicted r2 values: serum pegfilgrastim – 0.99; ANC – 0.95; band cells – 0.86; segmented neutrophils – 0.92.

Fig. 5:

Model predicted (lines) and observed (symbols) filgrastim serum concentration (left panel) and ANC (right panel) following the IV (5 μg/kg) and SC (1 μg/kg) of injections. Overall observed versus predicted r2 values: serum filgrastim – 0.80; ANC – 0.90.

Fig. 6:

In vitro model results. Time course measurements for control (left panel) and all end-of-assay measurements (right panel) following palbociclib treatment. Symbol: measurements, lines: model predictions. Overall r²=0.94.

Fig. 7:

Model predicted versus measures ANC values in all subjects (symbols). Line of identity (solid line). Regression line (dashed).

Fig. 8:

Model predicted versus measured ANC-time profiles in 12 selected patients from the four baseline ANC groups (three per group), representing the baseline ranges from below 3.3×10⁹ (first row), between 3.3–4.2×10⁹ (second row), 4.2–5.9×10⁹ (third row), and above 5.9×10⁹ cells/L (bottom row) of the measured baseline ANCs.

Fig. 9:

Data and model predictions for the two main dose groups: 125mg starting dose on a 3/1 schedule (left panel); 200mg starting dose on a 2/1 schedule (right panel). Model predictions for the median baseline ANC group 4.2×10⁹ cells/L – solid lines; model predictions for the 3.3×10⁹ cells/L baseline ANC group – lower dashed lines; model predictions of 5.9×10⁹ cells/L baseline – upper dashed lines. All measurements – open symbols.

Fig. 10:

For the 125 mg 3/1 schedule (left panel) and 200mg 2/1 schedule (right panel) dosing cohorts, the model-predicted percent of patients with grade 2, 3 or 4 neutropenia in each of the four baseline ANC groups. The gray scale reflects the percent of subjects in each column, (e.g., dark grey – greater than 50%, intermediate grey – 25% to 50%, light grey – less than 25%).