An in vivo mechanism for the reduced peripheral neurotoxicity of NK105: a paclitaxel-incorporating polymeric micellar nanoparticle formulation

Abstract

In our previous rodent studies, the paclitaxel (PTX)-incorporating polymeric micellar nanoparticle formulation NK105 had showed significantly stronger antitumor effects and reduced peripheral neurotoxicity than PTX dissolved in Cremophor^® EL and ethanol (PTX/CRE). Thus, to elucidate the mechanisms underlying reduced peripheral neurotoxicity due to NK105, we performed pharmacokinetic analyses of NK105 and PTX/CRE in rats. Among neural tissues, the highest PTX concentrations were found in the dorsal root ganglion (DRG). Moreover, exposure of DRG to PTX (C_{max_PTX} and AUC_{0-inf._PTX}) in the NK105 group was almost half that in the PTX/CRE group, whereas exposure of sciatic and sural nerves was greater in the NK105 group than in the PTX/CRE group. In histopathological analyses, damage to DRG and both peripheral nerves was less in the NK105 group than in the PTX/CRE group. The consistency of these pharmacokinetic and histopathological data suggests that high levels of PTX in the DRG play an important role in the induction of peripheral neurotoxicity, and reduced distribution of PTX to the DRG of NK105-treated rats limits the ensuing peripheral neurotoxicity. In further analyses of PTX distribution to the DRG, Evans blue (Eb) was injected with BODIPY^®-labeled NK105 into rats, and Eb fluorescence was observed only in the DRG. Following injection, most Eb dye bound to albumin particles of ~8 nm and had penetrated the DRG. In contrast, BODIPY^®-NK105 particles of ~90 nm were not found in the DRG, suggesting differential penetration based on particle size. Because PTX also circulates as PTX-albumin particles of ~8 nm following injection of PTX/CRE, reduced peripheral neurotoxicity of NK105 may reflect exclusion from the DRG due to particle size, leading to reduced PTX levels in rat DRG (275).

Keywords: dorsal root ganglion; micelle; paclitaxel; peripheral neurotoxicity; vascular permeability.

Figure 1

Concentration–time profiles of PTX after weekly injections of NK105 or PTX/CRE in rats. Notes: Concentration–time profiles of PTX in (A) plasma, (B) liver, (C) spinal cord, (D) brain, (E) DRG, (F) sciatic nerve, and (G) sural nerve tissues after six weekly injections of NK105 or PTX/CRE; data are presented as mean ± standard deviation. Abbreviations: DRG, dorsal root ganglion; PTX/CRE, paclitaxel formulated in Cremophor^® EL and dehydrated ethanol; PTX, paclitaxel.

Figure 2

Concentration–time profiles of the NK105 polymer and PTX after single injections of NK105 into rats. Notes: Concentration–time profiles of the NK105 polymer and PTX in (A) plasma and (B) the DRG after single injections of NK105. Abbreviations: DRG, dorsal root ganglion; PTX, paclitaxel.

Figure 3

Light micrograph of the DRG and the sciatic nerves in rats. Notes: The H&E-stained sections were prepared from (A) the DRG and (B) the sciatic nerve at 4, 8, and 10 days after the first injection of NK105 or PTX/CRE. NK105 and PTX/CRE were administered twice intravenously on days 0 and 3. Asterisk, degeneration of neurons; bright blue arrowheads, hypertrophy of satellite cells; yellow arrows, single cell necrosis of satellite cells; green arrow, mitotic satellite cells; black arrows, myelin ovoid. Abbreviations: DRG, dorsal root ganglion; H&E, hematoxylin and eosin; PTX/CRE, paclitaxel formulated in Cremophor^® EL and dehydrated ethanol.

Figure 4

Fluorescence immunohistochemical observations of markers for neural damage in the DRG and corresponding morphometric analyses in rats. Notes: Rats were injected with NK105 or PTX/CRE on days 0 and 3. (A) Fluorescent images of the neural injury marker ATF-3 show serial observations of DRG neurons at 4, 8, and 10 days after the first injection of NK105 or PTX/CRE. Yellow arrows, ATF-3-positive neurons; yellow arrowheads, ATF-3-positive satellite cells. (B) Immunohistochemical images of injury markers show serial observations in DRGs at 4, 8, and 10 days after the first injection of NK105 or PTX/CRE. (C) Percentages of ATF-3-positive and neurofilament-positive neurons; Ki-67-positive satellite cell numbers were serially calculated based on fluorescence immunohistochemical images of DRGs at 4 and 10 days after the first injection of NK105 or PTX/CRE. Abbreviations: ATF-3, activating transcription factor 3; DRG, dorsal root ganglion; PTX/CRE, paclitaxel formulated in Cremophor^® EL and dehydrated ethanol.

Figure 5

Fluorescence images of BODIPY^®-labeled NK105 polymer and Evans blue in rat DRGs. Notes: The H&E-stained sections and corresponding fluorescence images of DRGs were prepared at 4 and 24 h after a single injection of (A) BODIPY^®-labeled NK105 polymer or (B) Evans blue. Yellow dotted lines in (A and B) indicate DRGs. Abbreviations: DRG, dorsal root ganglion; H&E, hematoxylin and eosin.