ARTICLE ABSTRACTPurpose: Human epidermal growth factor receptor (HER) 1 and HER 1/2 inhibitors have shown benefit against a wide range of solid tumors. However, their use is associated with rash in 40% to 90% of patients, which impacts quality of life and interrupts antineoplastic therapy. The pathologic characteristics of affected skin remain unclear, precluding development of rational therapies. The aim of this study was to evaluate differences in histologic and immunohistochemical alterations in rash caused by lapatinib, a dual HER1/2 inhibitor (HER1/2i), and the single HER1 inhibitors (HER1i) cetuximab, erlotinib, and panitumumab.Experimental Design: For each of the four drugs, skin biopsies were collected and analyzed from 8 patients with rash (n = 32). Blinded independent histologic analysis and automated measurement of 17 skin biomarkers involved in proliferation, differentiation, and inflammation were conducted.Results: Increased expression of pAKT and decreased dermal K16 and p27 for HER1/2i when compared with each of the HER1i were observed. In addition, decreased epidermal atrophy and follicular neutrophilic infiltrate were evidenced in the skin of patients on HER1/2i when compared with HER1i.Conclusions: We found a lower inhibition of epidermal kinetics and decreased inflammation in HER1/2i-induced rash. These findings underscore differences in skin toxicity as related to specificity of HER blockade, concordant with clinical tolerability and decreased severity of skin toxicity seen with the HER1/2i lapatinib compared with the HER1 inhibitors cetuximab, erlotinib, and panitumumab. Clin Cancer Res; 16(17); 4452–60. ©2010 AACR.