Environment-Sensitive Polymeric Micelles Encapsulating SN-38 Potently Suppress Growth of Neuroblastoma Cells Exhibiting Intrinsic and Acquired Drug Resistance

Abstract

Conventional treatment approaches fail to provide durable control over aggressive malignancies due to intrinsic or acquired drug resistance characteristic of high-risk disease. SN-38, a potent camptothecin analog specifically targeting DNA topoisomerase I cleavage complexes, has shown promise in preclinical studies against aggressive solid tumors. However, its clinical utility is limited by inadequate solubility in pharmaceutically acceptable vehicles and by poor chemical and metabolic stability. Micelles formulated from amphiphilic invertible polymers (AIPs) can address these issues by concomitantly enabling solubilization of waterinsoluble molecular cargoes and by protecting chemically labile agents from inactivation. Furthermore, the inversion of the AIP and disruption of the carrier−drug complexes triggered by contact with cell membranes makes it possible to deliver the therapeutic payload into the cell interior without compromising its biological activity. In the present study, we characterized a novel AIP-based micellar formulation of SN-38 and evaluated its growth inhibitory effect on neuroblastoma (NB) cells derived either at diagnosis or at relapse after intensive chemoradiotherapy. Colloidally stable, drug-loaded micellar assemblies with a uniform <100 nm size were prepared using an AIP consisting of alternating blocks of poly(ethylene glycol) and polytetrahydrofuran (PEG 600 -PTHF 650 ). The micellar drug applied in a low nanomolar range (10−50 nM) completely suppressed the growth of chemo-nai ̈ve NB cells even after a brief (10 min) exposure. Furthermore, extending the exposure to 24 h resulted in a profound and lasting inhibitory effect of the micellar formulation on the growth of NB cells exhibiting an acquired loss of p53 function. These results suggest that micelle-mediated delivery of SN-38 can potentially offer a new and effective strategy for treating different phases of high-risk disease, including those showing poor response to conventional therapies.

Nanocarrier-Based Delivery of SN22 as a Tocopheryl Oxamate Prodrug Achieves Rapid Tumor Regression and Extends Survival in High-Risk Neuroblastoma Models

“…The importance of the rapidly activatable prodrug construction is evident from the robust killing effect of NP[SN22-TOx] on cultured IMR-32 cells, translating in vivo into rapid regression of IMR-32 orthotopic xenografts and markedly extended survival after five weeks of treatment with the NP/prodrug formulation at a low, non-toxic drug dose. The profound and lasting antitumor effects on IMR-32 cells and xenografts observed with SN22 in our study are in agreement with the relatively high chemosensitivity of MYCN-amplified NB cells derived pre-therapy [45,58]. However, a combination of MYCN amplification with acquired drug resistance in recurrent NB tumors results in a major shift in responsiveness to several families of chemotherapeutics, including topoisomerase I inhibitors [44,48].…”

“…In contrast, NP[SN22-TOx] administered weekly over four weeks caused rapid regression of the BE(2)C orthotopic xenografts and markedly reduced the rate of tumor regrowth, extending survival of most animals in this group beyond 26 weeks after the treatment initiation (Figure 6). In agreement with the highly chemoresistant phenotype of the BE(2)C cell line [45], irinotecan administered twice a week at 15 mg/kg was ineffective at inhibiting the growth of BE(2)C xenografts, with all animals reaching the endpoint within less than 5 weeks (Figure 6A,B). In contrast, NP[SN22-TOx] administered weekly over four weeks caused rapid regression of the BE(2)C orthotopic xenografts and markedly reduced the rate of tumor regrowth, extending survival of most animals in this group beyond 26 weeks after the treatment initiation (Figure 6).…”

“…The effect of prodrug-loaded NP and free SN22 on NB cell viability and growth was measured longitudinally as a function of drug concentration and exposure duration using firefly luciferase-expressing, MYCN-amplified cell lines, as previously reported [45]. IMR-32 and SK-N-BE(2)C cells purchased from the American Type Culture Collection (Manassas, VA, USA) were seeded at ~3500 cells/well on day −1 on 96-well plates using DMEM or DMEM/F-12 (1:1) supplemented with 10% FBS as the medium for the respective cell lines.…”

Poloxamer‐linked prodrug of a topoisomerase I inhibitor SN22 shows efficacy in models of high‐risk neuroblastoma with primary and acquired chemoresistance

“…Recurrent high-risk tumors with an acquired loss of p53 function exhibit a substantial reduction in chemosensitivity, requiring sustained drug presence in the tumor at levels exceeding by orders of magnitude those achievable clinically. 56 Thus, irinotecan capable of initially reducing the size of IMR-32 tumors showed only a marginal effect against BE(2)C orthotopic xenografts used in our study to model relapsed MYCN -amplified NB with an acquired p53 mutation. However, in contrast to the rapid loss of SN38 delivered as irinotecan with only trace levels detectable in the blood and tumor tissue after 24 h, a single dose of PF108-[SN22] 2 achieved stable drug presence over 72 h at levels at least 40 times higher than its concentration shown to stably suppress the growth of cultured BE(2)C cells.…”

“…Recurrent high‐risk tumors with an acquired loss of p53 function exhibit a substantial reduction in chemosensitivity, requiring sustained drug presence in the tumor at levels exceeding by orders of magnitude those achievable clinically 56 …”