(534e) Ultrahigh Drug-Loaded Nanoparticles for the Treatment of Triple-Negative Breast Cancer

Introduction: Triple-negative breast cancer (TNBC) is responsible for 15-20% of breast cancers and are unresponsive to therapies targeting estrogen, progesterone and HER2. Conventional chemotherapeutics are front-line for TNBC but resistance to these drugs make it a particularly challenging cancer to treat. Nanomedicine offers an exciting modality to better target, treat and potentially prevent resistance, e.g. by significantly increasing the maximum tolerated doses (MTD) of drug and consequently increasing the overall tumoral drug concentration. This effect is dependent on having a high drug loading capacity (LC), with ‘ultra’high drug loading (LC ≥ 35%) observed to give the highest potential MTDs. Herein, we have rationally designed a polymeric nanocarrier with a tunable, ROS-triggered release that are also capable of loading paclitaxel with an LC in the ‘ultra’high range; these particles were found to possessed field-leading MTDs of paclitaxel (150 mg/kg). We have further investigated the difference between fast or slow paclitaxel-release in a mammary fat pad model of TNBC.

Materials and Methods: A library of copolymers with tailored amounts of H-bonding (hydroxyl) and Ï€-Ï€ interacting (Benzyl, %B) groups were first synthesized via ring-opening polymerization. (A) ROS-responsive release rates were determined using Nile Red as a fluorescent surrogate in 1% H₂O₂ buffer. (C-D) MDA-MB-231 mammary fat tumors were established in nude mice and were treated once (day 0) or 5 time (day 0, 10, 20, 30 and 40) when tumors reached 50-100 mm³ ; slow=80%B, fast=40%B. Survival end-points were when tumors exceed 1000 mm³ or became ulcerated.

Results & Discussion: Polymer compositions with a higher benzyl composition (%B) presented slower and more sustained release profiles (Fig. 1A). We down-selected the 40%B and the 80%B as ‘Fast’ and ‘Slow’ release systems for further analysis as both also achieved a paclitaxel loading capacity of ~40 wt.% (‘ultra’high loading). Both were also found to have a field-leading MTDs of 150 mg/kg in mice treated 2x/week for 2 weeks; body masses all remained above 90% (Fig. 1B) and no toxicity was observed in blood biomarkers for the 150 mg/kg paclitaxel doses. In a xenograft mammary fat pad model of TNBC (MDA-MB-231s), using only a single dose of ultrahigh paclitaxel-loaded micelles at their MTD (150 mg/kg), the Slow-release group displayed a longer median survival than the Fast-release (60 vs 48 days) with 3/8 mice surviving the over 100 days (Fig. 1C-D). Both were significantly better than conventional paclitaxel treatment, Taxol and the saline control which had median survivals of only 34 and 26.5 days respectively. In a multidose regime (Fig.1E-F), mice were treated every 10 days for 40 days, the Slow-release outperformed the Fast-release formulation with 8/8 mice surviving for over 100 days (vs 7/8 Fast); a stable tumor remission was found in 4/8 mice (vs 3/8 Fast) and complete tumor elimination in 3/8 mice (vs 0/8 Fast).

Conclusions: A higher degree of benzyl in the polysulfide resulted in a slower drug release from the formulated micelles but also permitted ultrahigh drug loading (LC~40 wt.%); this corresponded to field-leading MTD of paclitaxel (150 mg/kg). A sustained (slow) release of paclitaxel from an ultrahigh-loaded nanoformulation (80%B polymer) resulted in an improved and more durable tumor response than a Fast-release formulation (40%B polymer).