In our previous study, we compared the serum 5-S-CD and LD levels as markers of MM, and reported that 5-S-CD was more efficient for detecting advanced-stage MM. In that study, the 5-S-CD levels were significantly higher in patients with stage III or IV disease than in patients with non-metastatic disease (stages 0 to II) [5]. However, in the present study, the 6H5MI2C and 5-S-CD levels were not significantly higher in the stage III group when compared to the non-metastatic group. One possible reason for this discrepancy is the differences between the MM populations analyzed in the two studies.
The method for measuring serum 6H5MI2C levels has just been established and cut-off values have not been determined [7]. In the present study, we examined the sensitivity of 6H5MI2C to each stage of MM at three different cut-off values (0.80, 0,90, and 1.00 ng/mL). With reference to the sensitivity of 5-S-CD and LD to MM in the non-metastatic group, a cut-off value of 1.00 ng/mL was considered appropriate for serum 6H5MI2C levels.
Among the three markers examined in the present study, the serum 6H5MI2C level appeared to best reflect MM progression. Similarly, Hara et al. previously reported that the plasma 6H5MI2C levels determined by HPLC reflected MM progression better and more reliably than the plasma 5-S-CD levels [10]. However, there have been papers showing that the serum and urinary levels of 5-S-CD reflect MM progression better than those of 6H5MI2C [11–13]. Although it remains to be determined whether 6H5MI2C or 5-S-CD is a better marker for MM, in the future, comparisons should be made with a larger number of cases.
Although various tumor markers for MM have been developed, only LD has been widely used worldwide as a serum biomarker. Serum LD is also included in the staging criteria for MM, and increased baseline LD levels reflect the prognosis of patients receiving nivolumab or pembrolizumab therapy [14]. LD can be measured with commonly available apparatuses for laboratory testing, and is thus relatively inexpensive and does not require specialized knowledge and skills to measure. In contrast, the HPLC for measuring 5-S-CD and the LC-MS/MS for measuring 6H5MI2C have the disadvantages of being expensive and requiring specialized knowledge and skills to perform, limiting their application. We have previously reported that the serum 5-S-CD levels may be a better predictive marker of nivolumab treatment efficacy than LD in patients with advanced MM [15]. Taken together with the results of the present study, serum melanin metabolites, especially 6H5MI2C measured by LC-MS/MS, appear to have the potential to be good markers for MM. Therefore, further experiments to examine their usefulness, optimize the measurement technology, and decrease costs are essential for the practical clinical application of melanin metabolites, including 6H5MI2C, as markers of MM.
Limitations
Samples from healthy volunteers are necessary to obtain precise cut-off values, sensitivity, and specificity of tumor markers. In the present study, however, the cut-off values and sensitivity of 6H5MI2C could only be determined using samples from MM patients only, because serum samples from healthy volunteers were not available.
Additionally, the present study compared the serum 6H5MI2C levels measured by LC-MS/MS to the serum 5-S-CD levels measured by HPLC, and it is possible that the difference in measurement methods affected the results; thus, the development of a technique for measuring the serum 5-S-CD levels by LC-MS/MS is also desirable.