Long-Acting Parenterals (LAPs) have been used in the clinic to provide sustained therapeutic drug levels at a target site, and thereby reducing the frequency of dosing required. In an effort to understand the factors associated with long-acting cabotegravir (GSK1265744 LAP) pharmacokinetic variability, the current study was designed to investigate the temporal relationship between intramuscular (IM) or subcutaneous (SC) drug depot morphology and distribution kinetics with plasma pharmacokinetics. Therefore, a multi-modal molecular imaging (MRI & MALDI IMS) approach was employed to examine the temporal GSK1265744 LAP biodistribution in rat following either IM or SC administration. Serial MRI was performed immediately post drug administration, and then at day 1 (24 h post), 2, 3, 4, 7, and 14. In a separate cohort of rats, an MRI contrast agent, Feraheme® (USPIO), was administered 2 days post IM drug injection in order to investigate the potential involvement of macrophages trafficking to the GSK1265744 LAP and Vehicle depot sites. The GSK1265744 LAP depot volume increased rapidly by day 2 in the IM injected rats (˜ 3–7 fold) compared with a ˜ 1 fold increase in the SC injected rats. In addition, the USPIO contrast agent labeled macrophages were shown to be present in the depot region of the GSK1265744 LAP injected gastrocnemius while the Vehicle injected gastrocnemius appeared to show reduced uptake. Matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS) of muscle and abdominal tissue sections identified the drug content primarily within the depot. Co-registration of the GSK1265744 ion images with immunohistochemical images established that the drug was taken up by macrophages associated with the depot. Linear regression analysis demonstrated that the drug depot characteristics including volume, surface area, and perimeter assessed by MRI at day 2 correlated with early time point plasma drug concentrations.
In summary, a multimodal molecular imaging approach was used to identify the drug depot location and volumetric/physiologic changes in both IM and SC locations following GSK1265744 LAP administration. The IM depot volume increased rapidly to a maximum volume at 2 days post-GSK1265744 LAP administration, while the Vehicle depot did not suggesting that the active drug substance and/or related particle was a key driver for drug depot evolution. The depot expansion was associated with an increase in macrophage infiltration and edema in and around the depot region and was correlated to plasma drug concentration at early time points (0–4 days). Consequently, molecular imaging approaches may be used in patients to help understand the biodistribution of GSK1265744 LAP and its associated pharmacokinetics.