Oseltamivir Population Pharmacokinetics in the Ferret: Model Application for Pharmacokinetic/Pharmacodynamic Study Design

<div><p>The ferret is a suitable small animal model for preclinical evaluation of efficacy of antiviral drugs against various influenza strains, including highly pathogenic H5N1 viruses. Rigorous pharmacokinetics/pharmacodynamics (PK/PD) assessment of ferret data has not been conducted, perhaps due to insufficient information on oseltamivir PK. Here, based on PK data from several studies on both uninfected and influenza-infected groups (i.e., with influenza A viruses of H5N1 and H3N2 subtypes and an influenza B virus) and several types of anesthesia we developed a population PK model for the active compound oseltamivir carboxylate (OC) in the ferret. The ferret OC population PK model incorporated delayed first-order input, two-compartment distribution, and first-order elimination to successfully describe OC PK. Influenza infection did not affect model parameters, but anesthesia did. The conclusion that OC PK was not influenced by influenza infection must be viewed with caution because the influenza infections in the studies included here resulted in mild clinical symptoms in terms of temperature, body weight, and activity scores. Monte Carlo simulations were used to determine that administration of a 5.08 mg/kg dose of oseltamivir phosphate to ferret every 12 h for 5 days results in the same median OC area under the plasma concentration-time curve 0–12 h (i.e., 3220 mg h/mL) as that observed in humans during steady state at the approved dose of 75 mg twice daily for 5 days. Modeling indicated that PK variability for OC in the ferret model is high, and can be affected by anesthesia. Therefore, for proper interpretation of PK/PD data, sparse PK sampling to allow the OC PK determination in individual animals is important. Another consideration in appropriate design of PK/PD studies is achieving an influenza infection with pronounced clinical symptoms and efficient virus replication, which will allow adequate evaluation of drug effects.</p></div

Monte Carlo simulation parameters and covariance matrix for the OC PK model.

<p>^a The CL and V parameters are conditioned on oral bioavailability and fraction of parent changed to metabolite.</p><p>CL_d = distribution clearance; CL_t = first-order clearance from the central compartment; K_a = first-order absorption rate constant; K_t = first-order transfer rate constant for transit compartments; OC = oseltamivir carboxylate; V_c = central compartment volume; V_p = peripheral compartment volume.</p><p>Monte Carlo simulation parameters and covariance matrix for the OC PK model.</p

OC PK parameters estimated in uninfected and influenza virus-infected ferrets.

<p>^a The CL and V parameters are conditioned on oral bioavailability and fraction of parent changed to metabolite.</p><p>CL_d = distribution clearance; CL_t = first-order clearance from the central compartment; CV = coefficient of variation; K_a = first-order absorption rate constant; K_t = first-order transfer rate constant for transit compartments; OC = oseltamivir carboxylate; PK = pharmacokinetic; Vc = central compartment volume; Vp = peripheral compartment volume.</p><p>OC PK parameters estimated in uninfected and influenza virus-infected ferrets.</p

Selected PK parameters and clinical signs in ferrets challenged with influenza A viruses (Study 1).

<p>^a OP dose was calculated based on free-base oseltamivir molecular weight.</p><p>^b Values are mean ± SD. All PD parameters were from morning observations.</p><p>^c Each ferret’s body temperature was measured by subcutaneous implantable encapsulated microchips (IPTT-300, Bio Medic Data Systems, US), which was recorded for 3 days before virus inoculation. The body temperature change was calculated for each ferret individually as an increase or decrease of its temperature on day 0 before virus inoculation. The values are averages for six animals per group (°C) ± SD. The average temperature decrease is indicated by minus. Positive average values indicate that the temperature increased. The number of ferrets per the total number of animals in that group that had at least a 1°C increase in temperature from baseline is shown in parenthesis.</p><p>^d The weight change was calculated for each ferret individually as a percentage of its weight on day 0 before virus inoculation. The values are averages for six animals per group (%) ± SD. The average weight loss is indicated by a minus sign. Positive average values indicate that animals were gaining weight. The number of ferrets per total number of animals in that group that had more than a 5% loss of initial weight on day 5 p.i. is shown in parenthesis.</p><p>^e The reported value is the mean morning activity score ± standard deviation observed on days 1–5 after inoculation on day 0. A score of zero indicated no nasal symptoms and a fully playful ferret. A score of one indicated nasal rattling or sneezing and a ferret that responded to play overtures but that did not initiate play. A score of two indicated nasal discharge and a ferret that was alert but not playful. A score of three indicated that mouth breathing was necessary and the ferret was not playful or alert.</p><p>^f Blood samples were not collected as OP was not administered.</p><p>AUC_ss12h = steady-state 12-h area under the plasma concentration-time curve calculated from the population PK model individual fit CL for each animal: AUC_ss12h = D×CL; CL = clearance; OC = oseltamivir carboxylate; OP = oseltamivir phosphate; PD = pharmacodynamic; p.i. = post-infection; PK = pharmacokinetic; SD = standard deviation; TCID₅₀ = 50% tissue culture infectious dose.</p><p>Selected PK parameters and clinical signs in ferrets challenged with influenza A viruses (Study 1).</p

Selected PK parameters and clinical signs in ferret challenged with influenza B virus (Study 2).

<p>^a OP dose was calculated based on free-base oseltamivir molecular weight.</p><p>^b Values are mean ± SD. All PD parameters were from morning observations.</p><p>^c Each ferret’s body temperature was measured by subcutaneous implantable temperature transponders (Bio Medic Data Systems Inc., Seaford, DE, United States of America), and was recorded for 3 days before virus inoculation. The values were averaged to obtain a baseline value. The body temperature change was calculated for each ferret individually as an increase or decrease of its temperature on day 0 before virus inoculation. The values are averages for 5–6 animals per group (°C) ± SD. The average temperature decrease is indicated by minus. Positive average values indicate that temperature increased. The number of ferrets per the total number of animals in that group that had at least a 1°C increase in temperature from baseline is shown in parenthesis.</p><p>^d The weight change was calculated for each ferret individually as a percentage of its weight on day 0 before virus inoculation. The values are averages for 5–6 animals per group (%) ± SD. Positive average values indicate that animals were gaining weight. The number of ferrets per total number of animals in that group that had more than a 5% loss of initial body weight on day 5 p.i. is shown in parenthesis.</p><p>^e The reported value is the mean morning activity score ± SD observed on days 1–5 after inoculation on day 0. A score of zero indicated no nasal symptoms and a fully playful ferret. A score of one indicated nasal rattling or sneezing and a ferret that responded to play overtures but that did not initiate play. A score of two indicated nasal discharge and a ferret that was alert but not playful. A score of three indicated that mouth breathing was necessary and the ferret was not playful or alert.</p><p>^f These data are from the group of ferrets administered a different dose on day 1, 3, and 5 to obtain PK data.</p><p>^g This group is the uninfected PK control that received a 0.76, 3.8, and 19.0 mg/kg OP dose on days 1, 3, and 5, respectively.</p><p>^h Blood samples were not collected as OP was not administered.</p><p>AUC_ss12h = steady-state 12-h area under the plasma concentration-time curve calculated from the population PK model individual fit CL for each animal: AUC_ss12h = D×CL; CL = clearance; OC = oseltamivir carboxylate; OP = oseltamivir phosphate; PD = pharmacodynamic; PFU = plaque forming units; p.i. = post-infection; PK = pharmacokinetic, SD = standard deviation.</p><p>Selected PK parameters and clinical signs in ferret challenged with influenza B virus (Study 2).</p

OC PK parameters estimated in ferrets with and without anesthesia.

<p>^a The CL and V parameters are conditioned on oral bioavailability and fraction of parent changed to metabolite.</p><p>CL_d = distribution clearance; CL_t = first-order clearance from the central compartment; CV = coefficient of variation; K_a = first-order absorption rate constant; K_t = first-order transfer rate constant for transit compartments; OC = oseltamivir carboxylate; PK = pharmacokinetic; V_c = central compartment volume; V_p = peripheral compartment volume.</p><p>OC PK parameters estimated in ferrets with and without anesthesia.</p

Monte Carlo simulation of steady-state OC PK at a 5.08 mg/kg OP dose.

<p>The thin solid curves are simulated PK for 100 individual ferrets, the open curves are the 10th–90th percentile, and the thick solid curve is the median OC concentration-time profile.</p

TFV quantification from rectal sponges is predictive of plasma TFV exposure during rectal dosing.

<p>Plasma TFV exposure is correlated linearly with rectal sponge TFV exposure. (p<0.001, robust RSE  = 0.38) The linear correlation is the same regardless of number of rectal doses. Shaded regions are the 10–90% confidence intervals of the mean predictions from robust linear regression model.</p