Thymidylate synthase expression determines pemetrexed targets and resistance development in tumour cells.

Although treatment options for cancer patients are increasing every year, the drug resistance problem remains very present. It is very difficult to find a drug that acts equally on tumours of the same histology as the individual's genetic characteristics often determine the response to treatment. Furthermore, tumours that initially respond to anti-tumour therapy are able to adapt and develop resistance to the drug, while others do not. In addition, this usually implies resistance development to agents to which the cells have not been exposed, a phenomenon called cross-resistance or multidrug resistance. Given this situation, it has been suggested that the most appropriate treatment would be able to act in parallel on multiple pathways constitutively altered in tumour cells. Pemetrexed is a multitargeted antifolate that exerts its activity against folate-dependent enzymes involved in de novo pyrimidine and purine synthesis. It is currently in use in combination with cisplatin against malignant pleural mesothelioma and non-squamous non-small cell lung cancer with favourable results. By real-time RT-PCR gene expression assays and restoration viability assays we demonstrated that Pemetrexed targets folate-dependent enzymes involved in de novo biosynthesis of purines differently depending on the intrinsic genetic characteristics of the tumour. These differences did not, however, interfere either with the initial response to the drug or with the activation of apoptotic pathways. In addition, these genetic fingerprints can differentiate two groups of tumours: those capable of developing resistance to antifolate, and not capable. These results may be useful to employ targets gene expression as resistance markers, a valuable tool for identifying patients likely to receive combination therapy to prevent the development of resistance

Table summarizing the genetic characteristics of studied cell lines.

<p>According to COSMIC database from <b>Sanger Institute</b> and “<b>The TP53 web site</b>”.</p

Viability and proliferation XTT after 48 h of exposure to MTA alone or in combination with dTh, Hx, or/and AICA.

<p>The percentage of viable cells is shown relative to viability of MTA-unexposed cells (control conditions). These results are representative of three independent experiments. <b>A)</b> Viability assays before and after MTA exposure with the pyrimidine biosynthesis pathway restored by addition of Hx alone or in combination with dTh. <b>B)</b> Viability assays before and after MTA exposure with purine biosynthesis pathway restored through the addition of AICA alone or in combination with dTh. <b>C)</b> Heatmap of six MTA-related genes where up- and down-regulation fold changes corresponding to each colour are indicated on the scale on the right of the figure.</p

A schematic diagram of the pyrimidine and purine biosynthesis pathways is shown, where the MTA-targets are indicated with a lightning bolt symbol.

<p>The effect of the addition to the culture medium of the preformed purine Hx, the preformed pyrimidine dTh and/or the purine pathway intermediate AICA as a new source for the restoration of the MTA-interrupted biosynthesis of purines and pyrimidines was different depending on the cell line.</p

MTA exerts its activity in different pathways increasing the apoptotic stimulus.

<p>Firstly, there is an accumulation of the AMP analogue ZMP that induces the activation of the AMPK pathway, starting a cascade of signalling that affects mTOR and PI3P/Akt pathways; mTOR is inactivated and the accumulation of its downstream unphosphorylated substrates facilitates the apoptosis process. Akt also remains inactive, unable to block p53 and to activate mTOR. On the other hand, the inhibition of TS, DHFR, GARFT and AICART induces oxidative stress and DNA damage which in turn is detected by p53 and caspase-dependent and independent mitochondrial apoptosis that is activated as has been previously reported. Together all processes lead to an imbalance between cell death and survival stimuli that result in enhanced apoptotic signalling.</p