These nanoparticles, termed nanobins, are composed of nanoparticulate arsenic trioxide encapsulated in liposomes. A second chemical layer provides protection for both the cargo and normal cells until the particle reaches its target. The nanoparticles concentrate at their target, as a consequence of the leaky blood vessels that characterise solid tumours, and release their toxic payload.
In the current study, published in Clinical Cancer Research, the researchers investigated the activity of arsenic nanobins against a panel of human breast cancer cell lines. Although less cytotoxic than free arsenic trioxide in vitro, the nanobins had dramatically enhanced efficacy in an in vivo model of triple-negative breast cancer. The scientists observed reduced plasma clearance, increased tumour uptake and induction of tumour cell apoptosis for the nanobins.
Triple negative breast cancer, in which the receptors for oestrogen, progesterone and Her2 are absent, is an aggressive cancer that often responds poorly to conventional chemotherapy. There is a high risk of metastatis and survival rates are low. Although at an early stage, the researchers anticipate that the nanobin technology could provide the means to increase the efficacy of a number of cytotoxic drugs against a range of tumours, whilst reducing general toxicity.
By far the majority of marketed drugs modulate the activity of proteins. Since the discovery of RNA interference (RNAi) over 10 years ago, a major goal has been to develop agents which will selectively block the synthesis of target proteins. Small interfering RNAs (siRNAs) occur naturally and are believed to have evolved as a defence mechanism against viral infection but synthetic siRNAs could potentially be used to silence any target gene. Although the idea is not conceptually difficult and many cancer-associated genes have been silenced in laboratory experiments, there have been a number of obstacles in developing RNAi for clinical use, not least of which is how to deliver the siRNAs to the target cells.
A team led by scientists and clinicians at the California Institute of Technology (Caltech) has now published the results of a small phase I clinical trial in patients with skin cancer showing that targeted nanoparticles can traffic into tumours and deliver siRNAs in a dose-dependent fashion when administered intravenously. The siRNA-containing nanoparticles used in the study are being developed by Calando Pharmaceuticals, Inc. and the technique used for detecting and imaging the nanoparticles inside cells from tumour biopsies was developed at Caltech. Although phase I studies are primarily designed as safety studies, the team was able to demonstrate that, in one patient who received the highest dose of nanoparticles, the target mRNA (M2 subunit of ribonucleotide reductase (RRM2)) had been cleaved at the predicted position and that protein levels had also been reduced.
Although more trials will be needed to show that such treatment is safe and effective, the study provides the first evidence that nanoparticles and RNAi can be combined to reduce expression of cancer-associated genes in human patients.