Abstract Purpose: Activating mutations in platelet-derived growth factor receptor-α (PDGFRA) have been reported in ∼5% to 10% of patients with gastrointestinal stromal tumors (GIST). Imatinib efficiently inhibits the juxtamembrane PDGFRA mutations, whereas many tyrosine kinase domain activation loop PDGFRA mutations confer primary resistance to imatinib. In this study, we compared the efficacy of second-line tyrosine kinase inhibitors such as dasatinib, sorafenib, and nilotinib against two GIST-related PDGFRA mutants, PDGFRAD842V and PDGFRAΔDIM842-844. In addition, we sought to investigate the inhibitory effect of the heat shock protein 90 inhibitor, IPI-504, on these mutants. Experimental Design: Primary imatinib-resistant tumor cells and cell lines expressing imatinib-resistant PDGFRAD842V or imatinib-sensitive PDGFRAΔDIM842-844 mutants were treated with different concentrations of dasatinib, sorafenib, nilotinib, and IPI-504. The effect of treatment on proliferation, survival, and signaling was determined. Results: All inhibitors tested exhibited a high efficacy toward the PDGFRAΔDIM842-844 mutant. In contrast, ex vivo and in vitro assays revealed that only dasatinib potently inhibited the PDGFRAD842V isoform with an IC50 value of 62 nmol/L. Sorafenib and nilotinib were significantly less efficacious against this mutation, inhibiting the PDGFRA kinase activity at >1,000 and >5,000 nmol/L, and suppressing the proliferation of the cells expressing the PDGFRAD842V mutant with an IC50 value of 239 and 1,310 nmol/L, respectively. IPI-504 treatment potently inhibited PDGFRA kinase activity by inducing the degradation of PDGFRAD842V and PDGFRAΔDIM842-844 at 256 and 182 nmol/L, respectively. Conclusions: Treatment with dasatinib or the heat shock protein 90 inhibitor IPI-504 may provide a therapeutic alternative for GIST patients whose tumors carry the imatinib-resistant PDGFRAD842V mutant isoform.