This investigation is the first to assess the association between MVR and antitumor efficacy of EGFR-TKIs in patients with NSCLC with EGFR mutations. This study showed a significant positive correlation between MVD and MVR, suggesting that MVR might be an indicator of angiogenesis, as well as MVD. Moreover, we found that high MVR in patients with EGFR mutation-positive NSCLC was a negative predictive factor for PFS of EGFR-TKI; thus, a novel treatment strategy for EGFR-TKI, such as a combination of EGFR-TKI and angiogenesis inhibitors, might be necessary in EGFR mutation-positive NSCLC with abundant tumor microvessels.
Numerous studies using MVD based on immunohistochemistry have shown that MVD is correlated with lung cancer development and prognosis (Bing et al., 2014; Meert et al., 2002; Zhang et al., 2021). In contrast, MVA has been reported to be a prognostic factor in breast cancer and renal cell carcinoma (Sato et al., 2014; Sharma et al., 2011; Sullivan et al., 2009). Moreover, Sullivan et al. (2009) described that the evaluation of MVA by automated image analysis was correlated with pathologist-diagnosed MVD (Sullivan et al., 2009), indicating that MVA is an alternative angiogenetic marker of MVD. However, MVA is measured as the total area of blood vessels per field of view and is dependent on magnification of the microscope and whole tumor size, which makes it difficult to consistently evaluate the overall assessment of angiogenesis status in whole cancer tissue. In the current study, MVR was defined as the ratio of microvascular area to field of view, likely to be estimated independently of tumor size, and was found to be positively correlated with MVD. Thus, the measurement of MVR using image analysis software could be an alternative indicator of tumor angiogenesis as well as MVD.
We found that the PFS of EGFR-TKI in the higher MVR group was significantly shorter than that in the lower MVR group. Tumor angiogenesis assessed by MVD has been reported to be associated with resistance to cytotoxic chemotherapy in several cancers, including breast, cervical, and ovarian cancers (Hasan et al., 2002; Li et al., 2021). One cause of drug resistance is the vascular endothelial growth factor (VEGF). VEGF plays an important role in tumor angiogenesis and leads to leakage from microvessels in tumor tissues and elevation of interstitial fluid pressure, which decreases drug delivery and results in drug resistance (Goel et al., 2012; Jain, 1994, 2013; Jain et al., 2014). Previous reports have shown MVD to be significantly correlated with plasma and tumor VEGF levels in NSCLC and esophageal and gastric cancers (Du et al., 2003; Tamura et al., 2001). Thus, EGFR mutation-positive NSCLC with higher MVR is suggested to be a microenvironment with abundant VEGF, which might reduce the efficacy of EGFR-TKIs due to inhibition of drug delivery.
Angiogenesis inhibitors, such as bevacizumab and ramucirumab, have demonstrated clinical efficacy and have been used for patients with NSCLC in routine practice. Recently, randomized phase III studies comparing the combination therapy of EGFR-TKI and angiogenesis inhibitors to EGFR-TKI monotherapy in EGFR mutation-positive NSCLC showed that the former resulted in longer PFS (Nakagawa et al., 2019; Saito et al., 2019). In ovarian and colorectal cancers, high MVD assessed by immunohistochemistry has been reported to be a potential predictive factor of bevacizumab (Bais et al., 2017; Bianconi et al., 2020). Furthermore, Zhao et al. (2012) reported a positive correlation between MVD and tumor shrinkage rate in patients with NSCLC treated with platinum doublets and bevacizumab (Zhao et al., 2012). We found that EGFR mutation-positive NSCLC with a high MVR was resistant to EGFR-TKI monotherapy compared to that with a low MVR. Based on these findings, EGFR mutation-positive NSCLC with abundant tumor microvessels might be preferable in combination with EGFR-TKI and angiogenesis inhibitors.
Despite these findings, our study had several limitations. First, this was a retrospective study performed in a single institution; therefore, various selection biases might be included. Second, this study used surgical specimens from patients with post-operative recurrence. The tumor microenvironment, including angiogenesis status, might have changed between the surgical and post-operative recurrent specimens, and these differences in microvasculature might have affected the results of our study. Third, the median value of MVR that we provisionally employed was cut off level in order to analyze the predictive factor of EGFR-TKI. Therefore, a large prospective study using advanced and post-operative recurrent NSCLC specimens is required to clarify the optimal cut-off value of MVR for the predictive value of response to EGFR-TKIs.