Study design and patient population
The popPK and covariate data analyzed in this study originated from a Phase I additional study of anlotinib, which evaluated pharmacokinetic, efficacy and safety in patients with advanced cancer confirmed pathologically and/or cytologically, with no standard therapy. Eligibility criteria included age between 18–65, Eastern Cooperative Oncology Group (ECOG) PS 0–1, and an estimated survival duration of more than 3 months. Patients administered other chemotherapy drugs halted their treatment for at least 30 days, while those who had received major surgery rested for at least 4 weeks [12]. The clinical study was conducted in accordance with the Declaration of Helsinki, International Conference on Harmonization, and Good Clinical Practice guidelines. The study protocol was reviewed and approved by the Institutional Review Board of Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, and informed consent was obtained from all patients [12].
Pharmacokinetic Sampling And Assays
A total of 16 patients (8 males and 8 females) were enrolled in the study. In the single dose phase, 13 patients administered a dose of 12 mg of anlotinib orally, while 3 received 16 mg. Ten days after the single dose, patients administered 12 mg of anlotinib once daily on a 2-weeks-on/1-week-off dosing schedule for two cycles of 21 days each. In the single-dose phase, dense blood samples were collected following administration at 0 h (pre-dose), and 1, 2, 4, 8, 11, 24, 48, 72, 120, 168, and 240 h post-dose. In the multiple-dose phase, blood samples were collected for each treatment cycle 24 h after dosing on days 1, 4, 7, 10, 14, and 18. One patient withdrew from the trial in the second cycle of multiple doses due to serious adverse events.
All anlotinib plasma concentrations were detected by a validated ultra-high performance liquid chromatography-tandem mass spectrometry method as previously described [13]. The lower limit of quantification (LLOQ) was 0.05 ng/mL. A total of 407 anlotinib concentrations were included for popPK analysis.
Poppk Analysis
The popPK analysis of anlotinib was conducted using nonlinear mixed-effects modeling (NONMEM) software (version 7.5, ICON Development Solutions). Covariate screening, bootstrap procedure, and visual predictive check (VPC) were performed using Perl-speaks-NONMEM (PsN, version 5.2.6, https://uupharmacometrics.github.io/PsN/). Data processing, calculation of descriptive statistics, and output visualization were performed using R (version 4.1.0, https://www.r-project.org/). The first-order conditional estimation method with interaction method was adopted for base model development and covariate testing. Model selection was based on the following: successful minimization; successful covariance estimation; objective function values (OFV); Akaike information criterion values; Bayesian information criterion values; precision of each parameter estimate; shrinkage for each interindividual and residual variability; condition numbers; visual inspection of scatter plots.
Anlotinib plasma concentration-time data was evaluated by one- or two-compartment models. The interindividual variability of the pharmacokinetic parameters was estimated using an exponential random effect of the form (Eq. 1):
$${\theta }_{i}=\theta \bullet {e}^{\eta i}$$
1
where θi represents the pharmacokinetic parameters of the ith subject, θ represents the typical value of the parameters, and ηi is defined as the random effect of the ith subject under the assumption of a normal distribution with a mean value of 0 and a variance of ω2, respectively.
The residual variability was characterized by the additive model (Eq. 2), proportional model (Eq. 3), and combined model (Eq. 4):
$${C}_{ij}={PRED}_{ij}+{\epsilon }_{1}$$
2
$${C}_{ij}={PRED}_{ij}\times (1+{\epsilon }_{1})$$
3
$${C}_{ij}={PRED}_{ij}\times (1+{\epsilon }_{1})+{\epsilon }_{2}$$
4
where Cij represents the jth observed concentration of the ith subject, PREDij represents the jth predicted concentration of the ith subject, \({\epsilon }_{1}\) and \({\epsilon }_{2}\) are defined as residual random variations on the assumption of a normal distribution with a mean value of 0 and a variance of ω12 and ω22, respectively.
Demographic and pathophysiological covariates were evaluated on the popPK of anlotinib. The continuous covariates evaluated included age, body weight, alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), alkaline phosphatase (ALP), γ-glutamyl transferase (γ-GGT), total bilirubin (TBIL), direct bilirubin (DBIL), indirect bilirubin (IBIL), total protein (TP), albumin (ALB), globulin (GLO), creatinine (CR), and urea nitrogen (UN). Sex and ECOG were evaluated as categorical covariates. An automated stepwise covariate model (SCM) building within PsN was used to identify significant covariates [14]. The correlation coefficient (r) between covariates was calculated, and one covariate was selected for analysis when r > 0.5. Covariates were then tested on each pharmacokinetic parameter with statistical criteria of P < 0.05 for forward inclusion procedure and P < 0.001 for backward elimination procedure. The effects of continuous covariates were estimated using a power function (Eq. 5), and the categorical covariates were estimated using a linear function (Eq. 6).
$$P={P}_{pop}\times {\left(\frac{COV}{median}\right)}^{{\theta }_{cov}}$$
5
$$P={P}_{pop}\times \left(1+{\theta }_{cov}\times COV\right)$$
6
where, P represents the individual pharmacokinetic parameter, Ppop represents the population value of the parameter, COV represents the value of the covariate, median represents the median value of the covariate, and θcov represents the slope of the covariate effect.
Model Evaluation
The final popPK model was evaluated using predictive checks and bootstrap analysis. Goodness-of-fit (GOF) plots were visually inspected. Visual predictive checks (VPCs) were performed 1000 times by simulating concentrations using the final model to validate the predictive performance. The bootstrap resample technique (n = 1,000) was used to calculate the standard errors and 95% confidence intervals (CIs) around the parameter estimates to validate the reliability and stability of the final model.
Simulation
Model-based simulations were performed 1000 times to evaluate the final covariate effects on the exposure of anlotinib. Area under the curve values from day 1 to day 21 (AUC0 − 21d) and Cmax of each patient with different covariate values were calculated. Each change in the AUC0 − 21d or Cmax was compared with a typical patient. The typical patient was defined by baseline medians (for continuous covariate) and modes (for categorical covariate) of the covariates in the total analyzed patients.