Release kinetics study of poorly water-soluble drugs from nanoparticles: are we doing it right?

Abstract

In vitro drug release kinetics studies are routinely performed to examine the ability of new drug formulations to modulate drug release. The underlying assumption is that the studies are performed in a sufficiently dilute solution, where the drug release is not limited by the solubility and the difference in release kinetics profile reflects the performance of a drug carrier in vivo. This condition is, however, difficult to meet with poorly water-soluble drug formulations, as it requires a very large volume of release medium relative to the formulation mass, which makes it challenging to measure the drug concentration accurately. These difficulties are aggravated with nanoparticle (NP) formulations, which are hard to separate from the release medium and thus require a dialysis bag or repeated high-speed centrifugation for sampling. Perhaps for these reasons, drug release kinetics studies of NPs of poorly water-soluble drugs are often performed in suboptimal conditions in which the NPs are not sufficiently diluted. However, such a practice can potentially underestimate drug release from NPs, leading to an inaccurate prediction that the NPs will attenuate the drug activity in vivo. Here we perform release kinetics studies of two different NP formulations of paclitaxel, a representative poorly water-soluble drug, according to common practices in the literature. We find that the drug release from NPs can be substantially underestimated depending on the choice of the release medium, NP/medium ratio, and handling of release samples. We discuss potential consequences of underestimating drug release, ending with suggestions for future studies with NP formulations of poorly water-soluble drugs.

Keywords: in vitro release kinetics; nanoparticles; paclitaxel; poorly water-soluble drugs; solubility; sustained release.

Figure 1

Paclitaxel (PTX) solubility in (a) PBS, (b) 0.2% Tween 80/PBS, and (c) 50% FBS/PBS. PTX solubility in each medium was determined by incubating excess PTX (0.6–2.4 mg) in 1 mL of medium at 37 °C for 7 or 24 h with agitation. Samples were centrifuged to remove precipitated PTX and analyzed with HPLC. (d) Stability of 1.5 μg/mL PTX in 0.2% Tween 80/PBS at 37 °C.

Figure 2

Release kinetics of PTX/NPs in media containing PBS, FBS, or Tween 80. PTX/NPs equivalent to (a) 4.4 μg or (b) 27 μg of PTX were suspended in 1 mL of release medium (PBS, Tween/PBS, or FBS/PBS) and incubated at 37 °C with constant agitation. At predetermined time points, the suspension was centrifuged to separate NP pellets and supernatants. Then 0.8 mL of supernatant was sampled and replaced with the same volume of fresh medium. The NP pellet was resuspended and returned for further incubation. The sampled supernatant was analyzed as sampled (PBS and Tween/PBS), with the addition of an equal volume of acetonitrile (PBS-AcN treated), or after extraction with ethyl acetate (FBS/PBS).

Figure 3

(a) Release kinetics of PNC and aPNC in PBS: PNC or aPNC equivalent to 200 μg of PTX were suspended in 3 mL of PBS, put in a dialysis cassette (MWCO 3500), placed in 200 mL of PBS, and incubated at 37 °C under constant agitation. At timed intervals, 5 mL of release medium was sampled and replaced with 5 mL of fresh PBS. (b) PTX concentration in the sampled medium at each time point. Symbols indicate each replicate. The dotted line indicates the saturation solubility of PTX in PBS.