a Hypomagnesemia and hypocalcemia are common adverse events during cetuximab treatment. The influence of the chemotherapeutic combination on serum levels is unknown and the predictive value is currently under discussion. This analysis investigated 79 patients who had received cetuximab for at least 6 weeks in the day clinic of the Comprehensive Cancer Center, University of Munich. Calcium and magnesium serum levels were analyzed weekly; tumor response and adverse events were followed. Thirty-eight patients had metastatic colorectal cancer (mCRC) and the predictive value of hypomagnesemia was tested in these patients. During therapy, calcium serum levels decreased to about 97% of the baseline levels and were maintained for the duration of treatment. Magnesium levels showed a significant time-dependent decrease. Serum levels of magnesium were lower when cetuximab was combined with a platinum derivative. After a treatment duration of 12 weeks, magnesium levels decreased to 70% in platinum-treated patients, whereas they decreased to only 90% of baseline in patients who did not receive platinum therapy. In patients treated for mCRC, a decrease of serum magnesium below 95% of the baseline levels 14 days after initiating treatment separated patients significantly in terms of survival times. Magnesium levels decrease in a time-dependent manner during cetuximab therapy. As hypomagnesemia was more prominent in patients receiving platinum agents, magnesium measurements may be advised in these patients. In mCRC patients treated with cetuximab, day-14 magnesium serum levels correlated with treatment efficacy.

521 Background: Hypomagnesaemia and hypocalcaemia are well known adverse events of cetuximab treated patients. Whether these side effects are of predictive value remains unclear. Methods: Patients receiving cetuximab for antitumoral treatment in the day clinic were investigated. Calcium and magnesium serum levels were analysed weekly during therapy, and tumor response and adverse events were followed. Serum levels were noted in relation to baseline measurements. Evaluable patients should at least be treated over a period of 6 weeks. Results: Between 2008 and 2011, a total of 97 patients were treated with cetuximab and were evaluable for serum levels. 38 patients had metastatic colorectal cancer, 27 suffered from SCCHN, 14 had other neoplastic diseases. During therapy, calcium serum levels decreased to a level of about 97% of baseline measurement and stayed there for the duration of treatment. Magnesium levels, however, showed a time dependent decrease during the whole observation time. When calculating the serum levels in relation to the chemotherapeutic combination partner, serum levels of magnesium were significantly lower when cetuximab was combined to a platin-derivative (oxali-, cis- or carboplatin). After a treatment duration of 12 weeks, magnesium levels dropped to 70% in platinum-treated pts, while they decreased to only 90% of baseline in pts without platinum therapy. In patients treated for metastatic colorectal cancer (mCRC) a decline of serum magnesium below 95% of baseline levels 14 days after initiating treatment separated patients with good outcome from those with poor outcome. While, a significant correlation between lower magnesium serum levels and the occurrence of acneiform exanthema was shown, no correlation of hypomagnesemia with clinical symptoms such as e.g. muscle cramps were documented. Conclusions: Magnesium levels decline in a time dependent manner during cetuximab therapy without occurrence of clinical symptoms. Hypomagnesaemia was more prominent in patients receiving platinum agents and was associated with better outcome.