Although the use of camptothecins in treating primary brain tumors has been widely researched, these compounds have yet to be used routinely in clinical practice, in large part because of their toxicity and related pharmacokinetic shortcomings.8,26
Liposomal encapsulation of chemotherapeutics has been shown to increase tumor drug exposure and to reduce systemic toxicity.27–30
Because of the size of therapeutic liposomes, previous brain tumor studies have often focused on their direct, local delivery to tumors by CED.31–34
Although nLS-TPT CED has shown efficacy in preclinical investigations using rodent models of GBM,14
enthusiasm for use of this route of administration has been restrained because of the invasive nature of CED and the limited success of CED-associated clinical trials conducted to date.35
In contrast to CED, intravenous delivery is less invasive and promotes a more uniform drug distribution throughout the brain, which is an important consideration because of the invasive nature of GBM. In our study, we investigated intravenous administration of nanoliposomal TPT for efficacy against 3 types of GBM xenografts and report for the first time, to our knowledge, that liposomal encapsulation enhanced TPT concentration in the brain, resulting in increased antitumor activity. We have previously shown that empty liposomes do not have antitumor activity,36
indicating that it is the liposomal packaging of TPT, and not the liposomes, that is responsible for improved antitumor activity in relation to free TPT.
By intercalating into DNA, TPT inhibits an essential process in proliferating cells, namely the role of topoisomerase I (TOPI) in DNA replication, ultimately resulting in DNA strand breaks that initiate programmed cell death. Indeed, the results of our TUNEL analysis show increased DNA strand breaks in TPT-treated tumors (Fig. ) and a corresponding increase in activated caspase-3 (Supplemental Figure 1
), a marker of programmed cell death. Of importance, DNA strand breaks and activated caspase-3 were significantly elevated by liposomal packaging of TPT. These results complement our previously published data indicating that TPT treatment depletes topoisomerase I in tumor cells36
and, in combination, address the mode of action and biologic consequence of TPT activity.
In contrast to targeted therapeutics, such as Tarceva, for which an antitumor effect has been shown to be specific to a subclass of GBM,37
TPT is expected to have a more generalized effect against GBM because of the essential nature of topoisomerase I in tumor growth. In fact, an analysis of The Cancer Genome Atlas (TCGA) data for DNA TOPI expression shows significantly elevated TOPI mRNA levels irrespective of GBM subclassification (Supplemental Figure 6
). Results from our analysis of 3 distinct GBM cell sources (U87-MG and serially propagated subcutaneous xenografts GBM6 and GBM43) for TPT treatment response (Figs and ) support the antitumor effect of this cytotoxic chemotherapeutic as being generalizable to most if not all subtypes of GBM.
The increased efficacy of nLS-TPT did not come at the expense of increased toxicity; there was not a significant difference in either mouse body weights or in neutrophil counts in mice treated with nLS-TPT, compared with mice treated with free TPT (Supplemental Figure 3
, Fig. ). Moreover, our analysis of blood cell counts in mice treated with TMZ followed by treatment with nLS-TPT indicate that nLS-TPT has a favorable safety profile for use in treating recurrent GBM.
Previous studies have not shown a significant advantage to adding TPT to radiation therapy.38,39
However, because of the dramatic improvements in distribution and efficacy seen with nLS-TPT, further study into nLS-TPT combined with radiation seems warranted. In addition, it would be of interest to examine the efficacy of TPT when used in combination with inhibitors of proteins that prevent apoptosis, such as obatoclax mesylate (GX15-070MS), a small-molecule pan-Bcl-2 family inhibitor recently shown to be well-tolerated when administered with TPT to patients with cancer.40
In total, our study results and previous results from others indicate several promising possibilities for maximizing benefit from intravenous administration of nLS-TPT and support additional investigation of this therapeutic agent and approach.