Promising Cancer RNAi Therapeutics Study Published in Cancer Research

Due to the relative ease of delivering RNAi effector molecules to the eye, lung, skin, and liver, it is expected that RNAi-based therapies for these organs will be the first to successfully navigate their way through the clinical trial maze. Nevertheless, a considerable amount of the early development efforts of RNAi Therapeutics is spent on cancer. I do not think that I need to remind anybody here about the need for more and better cancer therapeutics, but RNAi in particular offers the potential to greatly benefit cancer care. Delivery, however, represents a major obstacle for the development of cancer RNAi Therapeutics. This is even more so when faced with metastases which emphasises the need to find systemic RNAi delivery solutions to reach disseminated cancer cells throughout the body.

I was therefore very encouraged on reading a study published by Pirollo and colleagues from the Lombardi Comprehensive Cancer Center at Georgetown University Medical Center. In a Priority Report in the reputable journal Cancer Research (Cancer Res. 2007: 67 (7)), they report the development of a novel siRNA delivery system made up of nano-sized liposomes that harbour effector siRNAs and antibodies directed against the transferrin receptor. Transferrin receptors are known to be overexpressed on tumour cells and targeting them emerges as a promising strategy for cancer RNAi Therapeutics (Note: Calando, a subsidiary of Arrowhead Research, uses a similar approach in their first clinical RNAi cancer program). Consequently, the authors could show through imaging and nucleic acid detection techniques that accordingly formulated siRNAs were specifically enriched in the tumours of mouse models. The siRNA itself targets HER-2, a gene known to play a role in the survival of cancer cells. Although the siRNA in isolation already exhibited antiproliferative effects in tissue culture and the living animal, the most impressive data was that it could greatly sensitise cancer cells to the commonly used chemotherapeutic agent gemcitabine at low mg/kg doses. In addition to a dramatic reduction in tumour growth, the mice continued to gain weight and generally looked healthy. This study therefore shows the promise of using siRNAs in combination with chemotherapeutic agents, allowing for treatment regimens that are both more efficacious in addition to reducing debilitating side-effects.

Behind the success of these studies were 2 small tricks applied to the basic concept of nano-immunoliposomally formulated siRNAs. One was the addition of a pH-sensitive
peptide. This significantly enhanced cellular siRNA delivery, possibly by facilitating endosomal release of the siRNA cargo. The other trick was the modification of the siRNA, in which the non-targeting strand of the blunt-ended “siRNA” was greatly modified. This suggests that the systematic modification of basic siRNAs can be considerably advantageous and may yield even better siRNAs in the future.

Let us hope that other laboratories will find this immunoliposome technology equally promising so that it will eventually be tested in clinical trials.