Designing a killer to kill a killer

Unlike normal cells in our body, cancer cells keep on dividing and proliferating without control, forming lumps called tumours. If we find methods to stop such unrestricted growth, we can treat and cure cancer. Anti-cancer drugs are still not specific enough and often damage normal, healthy cells as well, leading to unpleasant side-effects. They also are not able to penetrate the tumour. ‘Surgical Strike’ or cutting off the tumour is not specific enough and tends to damage normal tissues as well. Physical tools such as nanoparticles, coated with ‘magic bullets’, which seek and destroy specific components of cancer cells, are limited by penetrability, clearance from the body after delivery and so forth. What is needed is a killer device that can open up the target cell mass and finish it off, and do so only on cancer cells and not harm healthy ones in the body.

Way back in 1893, Dr William Covey of New York thought up the unusual idea of using whole bacteria (or their extracts) for treating cancer. The argument is this: after all, these microbes enter our cells and wreak damage. He made extracts of microbes such as Mycobacterium bovis (one that causes TB in cattle) and found that tumours actually shrank in size upon treating with such ‘Covey toxins’.

Taming the shrew

This approach is, in effect, the use of one type of killer cells (microbes which cause disease to normal healthy cells, even death, if left untreated) to kill another type of killer cells (cancer). The trouble here is while they kill cancer cells, they may also damage healthy cells of the body; treating the body against such infection allows the cancer to come back. If only we find ways to ‘tame’ such microbes that they do not cause harm to normal cells, but specifically target cancer cells, we may have a winning strategy. While Covey could not do this, we now have ways to do so, thanks to the advances in genetic engineering, molecular and cell biology. The field of using bacterial cells, loaded with anti-cancer molecules, to fight cancer has grown fast in the last 20 years. The killer bacterium chosen by many researchers in the field is Salmonella typhimurium (the one that causes typhoid-like disease in rats, and leads to gastric problems and diarrhea in humans). The molecular biology of this pathogen (let us abbreviate its name as ST) is now well known, and genetically manipulating it is not difficult. A group of researchers at Yale University, Connecticut, USA, found that if we delete the gene called msbB from its genome, its toxicity towards normal cells is vastly reduced. However, although safe, the injection of this gene-deleted strain did not show substantial anti-tumour activity.

The group soon realised that compared to normal cells, tumour cells are far richer in ATP, the energy currency molecule in cells. Given this, if one were to delete the gene called purL (which codes for making the ‘A’ part in ‘ATP’) from ST, the modified strain would need external addition of ATP in order to multiply and grow. And the abundant levels of ATP are the teaser. Thus the purl- deleted ST strain would make a beeline towards the tumour cells, ignoring normal healthy ones. We thus have a second way of herding ST towards cancers.

What if we make a double mutant, that is, generate ST with both the msbB and purL genes deleted, grow and unleash it into the organ afflicted with cancer? This was indeed done about 15 years ago by a group of scientists from the pharmaceutical company called VION, near Yale University. This double mutant strain, termed V20009, was tried against mice with melanomas, and also on mice carrying human tumours grafted on to them. Intravenous injection of VNP20009 inhibited tumour growth anywhere between 57-95%. Plus, only live bacteria showed the anti-tumour effect, meaning that continuous infection by live bacteria is needed to eliminate tumours, and extracts or drugs with limited doses will not do. About the same time, a group from the National Cancer Institute, near Washington, DC, used VNP20009 to treat 24 human patients with secondary skin cancers and found it sufficiently safe for human use.

Making a package deal

Dr. Ravi Bellamkonda of Duke University, Durham, NC, USA went one step further, and decided to use VNP20009 as a carrier or vehicle, and loaded it with the protein called p53 which suppresses tumour growth, and another molecule called azurin which kills cancer cells, and also protects p53 from degradation. That such a twinning of p53 and azurin is useful was earlier shown by Dr. Ananda Chakrabarty of the University of Illinois College of medicine at Chicago.

Dr. Bellamkonda injected this cargo-laden ST on to rats which carried grafts of cancerous brain tumours obtained from humans. This therapy allowed the cancer-bearing rats survive for more than 100 days, compared with barely 26 days for untreated rats.

The beauty in using such cargo-laden double mutant pathogens is that (1) normal and healthy body cells are not affected, (2) they penetrate the body of the cancer cells, (3) this allows delivery of the drugs into the interior of the cancer cell (where conventional drugs find hard to enter), and (4) we can add more cargo, and allow additional cancer-killer drugs while keeping normal cells safe enough.

dbala@lvpei.org