Identification of novel combination-therapy drug targets, which work in synergy to mTOR inhibition using physiological human three-dimensional cancer models.

Carcinomas are the most frequent malignant tumours in humans, accounting for about 20.000 cancer deaths per year in Austria. In a so-called targeted therapy approach, scientists aim to find chemical inhibitors against specific molecules (proteins, often enzymes), which are aberrantly activated in human carcinoma to fight against cancer. One of these target proteins, the mammalian target of rapamycin (mTOR) has been identified as a key player in a cell-signalling pathway commonly over-activated in many human cancers (>50%). This has led to the assumption that inhibition of mTOR may be a useful tool to reduce tumour growth. Indeed, the specific mTOR inhibitor rapamycin is a potent negative regulator of cancer cell proliferation in tissue culture models. However, inhibition of mTOR by rapamycin displayed reduction of tumour growth in patients only in a few cancer types. Unexpectedly, in many other major cancers the inhibition of tumour growth was generally weak. The reasons for this poor anti-cancer activity are still rather unclear and need further investigation. Some new clinical studies use mTOR inhibitors in combination with either standard chemotherapy or other targeted therapy approaches. Some of these combination therapy trials show promising preliminary results with synergistic anti-tumour effects in vivo. The development of potential anti-cancer drugs mostly relies on tissue culture experiments performed on plastic culture dishes, also referred to as two-dimensional culture. However, most physio-logical parameters of organs or tumours such as tissue architecture, cell-cell interaction, mechanical properties and biochemical networks are lost under these simplified conditions. Cells grown as 3D aggregates (multicellular spheroids) much better recapitulate the in vivo situation of three-dimensional tumours. Importantly, multicellular tumour spheroids display drug response and resis-tance characteristics closely matching those regularly seen in vivo. Consequently, these preclinical models might be physiological more relevant to test potential therapeutic targets. We have developed such a 3D cell culture system to monitor the growth of cancer cells in vitro, which can be used to test hundreds of chemical compounds for their anti-tumour activity in a physi-ologic setting. We will use this cellular screening tool to systematically search for chemical com-pounds, which act synergistically to rapamycin and will together have an amplified growth inhibitory or toxic effect on cancer cells. Importantly, we will use a collection of 1200 chemical compounds, which is commercially available and contains only already approved drugs for the clinic to re-evaluate these known drugs for their potential anti-tumour efficacy when combined with rapamycin. The use of existing drugs for new therapies is advantageous in many respects, such as the knowledge of the effective drug levels and known side effects. 24 existing drugs are already being reused for new indi-cations. In addition to the identification of new effective drug combinations we will thereby discover novel pathways, which drive cancer development and will focus on the molecular mechanisms be-hind.