(732e) Interstitial Release of Cisplatin from Triggerable Liposomes Enhances Efficacy Against Triple Negative Breast Cancer Solid Tumor Analogues

In solid tumors, including Triple Negative Breast Cancer (TNBC), limited penetration of therapeutics leads to heterogeneous intratumoral distributions reducing efficacy. Liposomal cisplatin decreases toxicities associated with free cisplatin. However, the heterogeneous intratumoral distributions of liposomes combined with limited release of cisplatin from liposomes may also reduce efficacy. This study uses pH-responsive liposomes designed to release cisplatin within the acidic tumor interstitium (7.0 < pH â?¤ 6.0) with the aim (1) to improve the penetration of the free drug within tumors on the assumption of greater diffusivities based on the free drug's much smaller size and (2) to increase the availability of the free agent near cancer cells.

Introduction: In advanced solid tumors, including Triple Negative Breast Cancer (TNBC) solid tumors, the limited penetration of therapeutics results in limited therapeutic effects. TNBC is a subgroup of breast cancer associated with a high chance of cancer reoccurrence outside the breast and has been found to exhibit sensitivity to DNA-damaging platinum-based compounds, which is the basis for increased clinical use of platinum-derived agents. To enable selective and effective treatment of TNBC solid tumors, we study a drug delivery carrier of CDDP that is expected to result in potentially deep penetration and homogeneous distribution of the drug within the tumor interstitium. The carrier is a tunable (pH-releasing) liposome encapsulating CDDP. Tumor selectivity by these liposomes is mediated by the EPR effect. Efficacy is mediated by the pH-triggered release of contents within the tumor interstitium (7.0 < pH < 6.5) by extravasated liposomes. The small size of CDDP relative to its nanometer-sized carrier should exhibit greater diffusivities and deeper tumor penetration.

Methods: To study the efficacy of these liposomes on TNBC cells two different TNBC cell lines were studied: MDA-MB-231 and MDA-MB-468. The efficacy of pH-releasing liposomes (LD₅₀) loaded with CDDP was compared to the efficacy of other constructs including conventional (non-p-releasing) liposomes, empty liposomes, and free cisplatin. Following incubation of 6 and 24 hours, in addition to cell viability, cisplatin per cell, cisplatin intercalated to DNA and (if applicable) retention of cisplatin within the liposomes, were measured. In multicellular spheroids exhibiting intracellular pH gradients measured in situ by confocal microscopy using a membrane impermeant pH-indicator, we evaluated the spatiotemporal distributions of liposomes and contents. In dose-response studies, upon completion of incubation with all constructs, spheroids were washed and their volume change (volume at time t/volume at initiation of treatment x100%) was monitored in time followed by an outgrowth assay.

Results: On MDA-MB-231 and MDA-MB-468 cell monolayers, extracellular acidification resulted in lower LD₅₀ values only for pH-releasing liposomal cisplatin (101.9 ± 24.1 and 14.5 ± 3.6 µg/mL, respectively) which were significantly less than for non-pH-releasing liposomal cisplatin (374.2 ± 20.0 and 33.7 ± 5.0 µg/mL, respectively). In contrast, at neutral pH, the LD₅₀values of both liposome types were comparable. Killing efficacies correlated strongly with the nuclear DNA-associated cisplatin independent of the delivery modality or the extracellular pH. pH-releasing liposomal cisplatin decreased the volumes of multicellular spheroids by 46.4-70.0 % more than non-pH-releasing liposomal cisplatin. pH-releasing liposomes resulted in improved penetration and more homogeneous profiles within spheroids of a cisplatin-surrogate cell-permeant fluorophore compared to non-pH-releasing liposomes.

Conclusions:These findings demonstrate the potential of intratumorally CDDP-releasing nanometer-sized carriers to selectively and effectively control TNBC solid tumors.

Funding Sources:

American Cancer Society (ACS): RSG-12-044-01

National Science Foundation (NSF): DMR-1207022

AAUP-AFT Professional Development Fund