We observed that encapsulation of As2
in a novel nanobin, NB(Ni,As), led to increased in vivo
antitumor efficacy of As2
in the MDA-MB-231 orthotopic model of human triple negative breast cancer. NB(Ni,As) inhibited mammary tumor growth at doses (4 mg/kg twice weekly) that are significantly lower than the anticipated efficacious dose of the parent drug based on published reports in other preclinical models of solid tumors (36
). Indeed, two times the equivalent dose of free As2
had no effect on tumor growth in our study (data not shown). We attribute the enhanced antitumor efficacy in vivo
of NB(Ni,As) to the reduced plasma clearance and the increased tumor accumulation of arsenic via the EPR effect of the nanobin platform. Specifically, we observed that the area under the curve (AUC∞
), an estimate of drug exposure, was increased approximately 300-fold for NB(Ni,As) compared with free As2
. Moreover, the tumor arsenic concentrations were 3–5 fold higher in NB(Ni,As) treated mice. The As2
that is delivered to tumors by NB(Ni,As) appears to be released over an extended time period, leading to metronomic-like dosing compared to free As2
and resulting in prolonged antitumor activity even after treatment was stopped (). In contrast, free As2
had no anti-tumor activity at the dose and schedule used and may have caused a “rebound” tumor growth. The latter phenomena has been observed in maximal tolerated dose (MTD)-based regimens where neovascularization and tumor growth resumes during the necessary recovery periods from drug-induced toxicity (38
). The NB(Ni,As) may block this tumor rebound effect by loading a tumor with drug that is continuously released and does not follow the peak-trough kinetics typically associated with MTD-based chemotherapy. To that end, NB(Ni,As) may behave like a metronomically dosed or depot agent, although that aspect of NB(Ni,As) pharmacology remains to be explored.
The inhibition of NB(Ni,As)- and free As2
-induced cell death by Z-VAD-FMK, and the enhanced caspase-3 activation in mammary tumors observed in mice treated with these cytotoxic agents, indicate that the antitumor effects of NB(Ni,As) are at least partly mediated by a caspase-dependent apoptotic mechanism. Although we observed similar rates of mammary tumors apoptosis in the NB(Ni,As) and free As2
groups 48 h after treatment, only NB(Ni,As) reduced tumor burden in vivo
. This seeming paradox likely reflects the rapid clearance of free As2
from the circulation, leading to transient induction of apoptosis in the mammary tumors and potential “rebound” growth (36
). In contrast, NB(Ni,As) prolongs the pharmacokinetics of As2
and increases tumor uptake, resulting in sustained antitumor effects. Intriguingly, free As2
and NB(Ni,As) inhibit migration and invasion at concentrations well below their IC50
values, suggesting that these agents may have anti-metastatic activity in addition to their cytotoxicity.
is currently approved for use in acute promyelocytic leukemia (APL), several clinical trials in patients with solid tumors have failed to demonstrate a clinical benefit of As2
at doses in the 0.25–0.35 mg/kg/d range (39
). APL patients receive 0.16 mg/kg/d of As2
, and this dose is associated with Grade 3/4 toxicities such as peripheral neuropathy, hepatic and cardiac toxicity (10
). These dose-limiting toxicities have limited further dose escalation of As2
in other malignancies. Conversion between mouse and human dose levels by body weight (mg/kg) can be estimated by dividing the mouse dose by 12.3 (41
). Thus, a 4 mg/kg dose in the mouse is estimated to be equivalent to a human dose of 0.32 mg/kg, which results in a comparable weekly dose of free As2
used in APL patients because the nanobin is given twice weekly rather than daily. Hence, clinically efficacious concentrations of NB(Ni,As) are associated with minimal systemic toxicity in this preclinical model.
As noted, we observed that NB(Ni,As) is more effective at suppressing tumor growth in vivo than the parent drug As2O3; however, free As2O3 is much more cytotoxic in vitro. This disparity suggests that standard in vitro cytotoxicity assays of nanoparticle-encapsulated drugs may be a poor predictor of in vivo antitumor activity because they fail to capture the effects of drug encapsulation on pharmacokinetics and biodistribution in vivo. Hence, early in vivo testing of nanoparticle encapsulated cytotoxic agents in animal models of cancer is of paramount importance and an important component of the development of this class of drugs.
Many potentially effective cancer drugs have been abandoned due to unacceptable systemic toxicity, poor pharmacokinetics and/or biodistribution (22
). We have devised a nanoparticulate platform (nanobin) in which the nanoscale size leads to the concentration and retention of drug in the tumor, while the nanobin encapsulation improves the pharmacokinetic characteristics and potentially the toxicological properties of the encapsulated drug. This platform has been applied to the encapsulation and reformulation of a highly cytotoxic drug, As2
, which is currently limited in its use clinically to APL and other hematologic malignancies. The reformulated As2
nanobin, NB(Ni,As), differs substantially from parent drug in both its pharmacological and efficacy profile. These results provide a foundation for additional preclinical development and future clinical interventions in breast cancer and other solid tumors.
Statement of Translational Relevance
Arsenic trioxide (As2O3) is a highly effective therapy for acute promyelocytic leukemia and has antitumor activity in preclinical models of solid tumors; however, clinical trials of As2O3 in several solid tumors reveal a narrow therapeutic window that limits wider application. Nanoscale drug carriers have increased the therapeutic index of several cytotoxic agents by increasing tumor drug delivery, enhancing antitumor efficacy and attenuating systemic toxicity. We have developed a novel high-density nanoparticulate formulation of As2O3 that is encapsulated in 100 nm liposomal vesicles or nanobins. Nanobin encapsulation of As2O3 [NB(Ni,As)] dramatically improves pharmacokinetic properties of the active agent and leads to greater therapeutic efficacy compared to free As2O3 in an orthotopic model of triple negative breast cancer. Moreover, we show that NB(Ni,As) is well tolerated in this model, suggesting that this nanoscale platform may have the potential to expand the clinical utility of As2O3 as a cancer therapeutic agent.