2028 Background: KOS-862 (Epothilone D) (K) has shown single agent activity against metastatic breast cancer (MBC) and NSCLC. Trastuzumab (H) is standard therapy for HER-2 overexpressing MBC. This phase IB trial determined the safety and PK profile of KOS-862 in a weekly schedule with H. Methods: Eligibility: pts with HER-2 overexpressing (IHC 3+ or FISH +) chemo-refractory tumors, ECOG ≤ 2, neuropathy ≤ gr 1, no CNS metastases, and no H ≤ 6 mo. prestudy. H given as a 4 mg/kg loading dose, then 2 mg/kg wkly. K given as a 90 min iv infusion wkly × 3 Q 4 wks after H. K dose escalation cohorts: 70, 85 & 100 mg/m2. H1 + H2 antagonists + corticosteroid prophylaxis given 30–60 min prior to K. DLT: gr 4 hematologic or gr 3 other toxicity, or toxicity requiring >4 wk treatment delay. Cycle 1 + 2 PK samples obtained for K + H. Results: 13 pts treated from 9/04 - 8/05. Median age 51 yrs (36–65). All pts had MBC. Prior H therapy n = 9, prior taxane n = 11. Enrollment: 70 mg/m2 (4), 85 mg/m2 (3), and 100 mg/m2 (6). Mean number cycles 3.9 (1–6). The 100 mg/m2 cohort was expanded to 6 pts per study design; no DLTs were noted. Toxicity: Principal neurotoxicities (gr 3 noted, if present): parasthesias (10 pts, 1 gr3), neuropathic pain (5 pts, 2 gr3), dizziness (4 pts), dysguesia (4 pts), ataxia (3 pts), transient confusion (3 pts), dysesthesias (2 pts), insomnia (2 pts). No gr 4 neurotoxicities reported. Non-neurologic toxicities were limited to gr 1/2 and included diarrhea, pyrexia, abdominal and back pain, arthralgias. PK (n = 13): KOS-862 CL 21 ± 10 L/hr, Vz 260 ± 110 L, T1/2 9.8 ± 3.9 hr. Seco-metabolite was 67% ± 35% of K. At 100 mg/m2 K dose (n = 6): Cmax 3.3 ± 1.1 μg/mL and AUCinf 11.0 ± 7.5 h*(g/mL). Neither K nor its seco-metabolite PK were affected by H. Responses: An 85 mg/m2 K pt (no prior H or taxane) had confirmed PR and a 100 mg/m2 K pt (prior taxane) had an unconfirmed PR. A 2nd 100 mg/m2 pt (no prior H or taxane, bone only disease) had > 90% CEA reduction. Conclusions: This phase I study demonstrates feasibility of full dose wkly KOS-862 coadministration with wkly H. Treatment-related toxicities were all gr 1 / 2, except for 3 cases gr 3 paresthesias and neuropathic pain. PK of K and its principal metabolite were not affected by H. Three clinical responses were observed; a phase II trial of this combination is ongoing in minimally-pretreated MBC. [Table: see text]