Homogeneity of mesothelin expression is tightly linked to its levels of expression in non-small cell lung cancer

Mesothelin is a membrane protein which is commonly expressed in lung cancer as well as in other cancer types while it is only rarely seen on the surface of normal cells. Therefore, mesothelin represents an attractive molecule for targeted cancer therapies employing CAR-T cells, monoclonal antibodies, recombinant immunotoxins, antibody-drug conjugates and other drugs. For targeted therapy, both the therapeutic success and the quality of the diagnostic assessment generally may critically depend on the degree of heterogeneity of target protein expression in each individual cancer. Cancers with mesothelin expression in all cancer cells are most likely to be correctly diagnosed as “mesothelin positive” in small biopsies and may also respond optimally to therapy as all tumor cells carry the critical target signature. To study the extent of intratumoral heterogeneity of mesothelin expression in non-small cell lung cancer (NSCLC), a NSCLC heterogeneity tissue microarray was analyzed by immunohistochemistry (IHC) in a tissue microarray (TMA) format. The TMA contained 0.6 mm tissue samples from 8 areas each of the primary cancers of 130 patients and up to 4 samples (1 sample per nodal metastasis) from matched lymph node metastases from 60 of these patients. Mesothelin positivity was observed in 411 of 1,007 (40.8%) evaluable samples. On a patient level, mesothelin positivity occurred in 56.2% (n=73) while the highest mesothelin staining intensity was strong in 30.0% (n=40), moderate in 9.2% (n=12), and weak in 16.2% (n=21) of patients. Heterogeneity analysis was based on 7.7 samples per patient on average (range 2-12) and revealed a homogeneous mesothelin positivity (observed in all evaluable samples) in 21.5% (n=28), a heterogeneous mesothelin positivity in 34.6% (n=45), and a homogeneous lack of mesothelin staining in 43.8% (n=57) of patients. Homogeneous mesothelin positivity was tightly linked to a high level of mesothelin expression. Among 73 patients with detectable mesothelin positivity in their cancers, positivity was homogeneous in 67.5% (27 out of 40) of patients with at least one strongly positive sample, 8.3% (1 out of 12) with only moderate positivity and not seen (0 out of 21) in patients with only weak mesothelin positivity (p<0.0001). It is concluded that NSCLC patients with high level mesothelin expression do mostly exhibit a homogeneous positivity across their cancer while the likelihood for heterogeneous findings increases in patients with a more equivocal mesothelin positivity. These findings could further support the concept that a high level of mesothelin staining could be related to a favorable response to anti-mesothelin drugs in NSCLC patients.