Summary:

Metastatic and multidrug resistant (MDR) tumors are generally very challenging to treat; and while in many cancers early detection can increase the survival rate significantly, the mortality rates of advanced and metastasized cancers are still quite high. Understandably, this drug resistance greatly limits treatment options. Overexpression of P-glycoprotein (Pgp) or β-III tubulin highlight two important mechanisms responsible for drug resistance. To have therapeutic efficacy in MDR tumors, the drug molecule should be able to bypass these mechanisms. Podophyllotoxin (PPT) is one such compound that, according to previous studies (Roy et al. 2015), is significantly active in Pgp-mediated MDR cell lines. PPT however, can be highly toxic and is poorly soluble and therefore cannot be applied systemically. Using the NanoAssemblr^TM Benchtop instrument, the Shyh-Dar Li group from the University of British Columbia have developed a polymeric nanoparticle formulation of PPT, named Celludo, that can be intravenously administered at significantly higher doses than the naked PPT with minimal toxicity. Due to the leaky tumor vasculature, these nanoparticles are readily taken up by tumor tissue and are efficacious in prolonging the survival of mice implanted with metastatic MDR tumors.

In their article featured in Biomaterials, 2017, The Li group extensively investigated pharmacokinetics and efficacy of Celludo. Unlike naked PPT with maximum tolerated dose of 20 mg/kg in mice, Celludo can be injected at doses as high as 180 mg/kg PPT. Furthermore, no histological lesions in any tissues were observed in non-tumor bearing mice injected with Celludo and none of the typical PPT side effects (neurological disturbance, vomiting, diarrhea, and abnormal hepatic functions) occurred with Celludo at 180 mg/kg PPT. The authors then investigated the pharmacokinetics and biodistribution of Celludo in tumor bearing mice. The results indicated a significantly prolonged half-life and 56-fold tumor delivery of Celludo compared to free PPT. It was speculated that the high tumor accumulation of Celludo was partly due to its small size of 20 nm, allowing for enhanced permeability and retention (EPR). However, heterogeneity in clinical tumors and the existence of poorly vascularized tumors can lead to reduced EPR effect. For these reasons, the nanoparticles solely relying on passive targeting (EPR) poorly accumulate in small metastatic nodules. The Li group therefore, established MDR lung nodules in mice and then i.v. injected them with fluorescently labeled Celludo. Interestingly, imaging of tissue indicated that Celludo accumulated in lungs and specifically in MDR nodules. Very little fluorescent signal was detected in the liver tissue (about 1/8 of lung), pointing to target specificity of Celludo and making it the first non-ligand targeted nanoparticle to specifically target lung metastasis.

Efficacy of Celludo was assessed in a murine metastatic breast cancer model. Standard therapies for metastatic breast cancer consist of taxanes and doxorubicin, with lung as the major organ of metastasis. Of the mice bearing MDR breast tumors, those treated with Celludo had significantly prolonged survival time by 20 days in comparison to the mice treated with saline, docetaxel or naked PPT. Celludo-treated mice also exhibited minimal weight loss, no visible sign of distress and reduced lung tumor burden. Celludo was also tested in a murine metastatic MDR ovarian cancer model. Ovarian cancer is usually diagnosed at advanced stages where very few intervention options are available and the mortality rates are very high due to aggressive metastasis. MDR ovarian tumor-bearing mice were treated with PEGylated liposomal doxorubicin, paclitaxel (the two common treatments for this condition), saline, PPT and Celludo. Celludo prolonged mean survival time by 20 days compared to the other treatment groups.

In conclusion, Celludo is an efficacious means of PPT delivery to metastatic MDR tumors. While PPT is a potent anticancer agent and can bypass the drug resistance mechanisms, it has poor solubility and is highly toxic. Celludo nanoparticles can be dosed much higher than the naked PPT because they reduce off-target toxicity, thus widening the therapeutic window. This study demonstrates how nanotechnology can be used to salvage otherwise impractical drugs, and offer hope for future treatment of metastatic cancers.

Abstract: 

Treatment options for metastatic and multidrug resistant (MDR) tumors are limited, and most of the chemotherapeutic drugs exhibit low efficacy against MDR cancers. An anti-tubulin agent podophyllotoxin (PPT) displays high potency against MDR tumor cells. However, due to its poor solubility and non-specificity, PPT cannot be used systemically. We have developed a self-assembling nanoparticle dosage form for PPT (named Celludo) by covalently conjugating PPT and polyethylene glycol (PEG) to acetylated carboxymethyl cellulose (CMC-Ac) via ester linkages. Celludo displayed extended blood circulation with an 18-fold prolonged half-life (t1/2), 9000-fold higher area under the curve (AUC), and 1000-fold reduced clearance compared to free PPT. Tumor delivery was 500-fold higher in the Cellduo group compared to free PPT. Against the lung metastatic model of EMT6-AR1, Celludo showed selective localization in the metastatic nodules and increased the median survival to 20 d compared to 6–8 d with docetaxel and PPT treatment. In the intraperitoneal metastatic model of human ovarian NCI-ADR/RES tumor, Celludo prolonged the median survival from 50 d to 70 d, whereas the standard therapy PEGylated liposomal doxorubicin showed no effect. No major toxicity was detected with the Celludo treatment. These results demonstrate that Celludo is effective against metastatic and MDR tumors.