Phase I dose escalation study of 12b80 (hydroxybisphosphonate linked doxorubicin) in naturally occurring osteosarcoma

Abstract

Purpose: 12b80 combines doxorubicin bound to a bone targeting hydroxybisphosphonate vector using a pH-sensitive linker, designed to specifically trigger doxorubicin release in an acidic bone tumor microenvironment. This phase I study aimed to determine the safety and toxicity profiles of 12b80 in dogs with naturally occurring osteosarcoma, with the objective to translate findings from dogs to humans.

Experimental design: Ten client-owned dogs with osteosarcoma were enrolled in an accelerated dose-titration design followed by 3 + 3 design. Dogs received three cycles of 12b80 intravenous injection at 4 mg/kg (n = 1), 6 mg/kg (n = 2), 8 mg/kg (n = 3), and 10 mg/kg (n = 4). Endpoints included safety, tolerability, maximum tolerated dose (MTD), and dose-limiting toxicity (DLT).

Results: The MTD of 12b80 was 8 mg/kg (i.e., equivalent dose of doxorubicin of 110 mg/m², range: 93-126). Most adverse events included grade ≤ 2 gastrointestinal disorders and hypersensitivity reactions. No hematological or cardiac DLT were observed at any dose tested.

Conclusions: In dogs, 12b80 is overall well tolerated and expends the MTD of doxorubicin up to four times the standard dose of 30 mg/m². These results demonstrate the potential therapeutic benefit of 12b80 in canine and human osteosarcoma.

Keywords: bisphosphonate; bone targeting; canine; doxorubicin; osteosarcoma.

Figure 1. Circulating blood cell counts in dogs treated with 12b80.

The protocol included three cycles of 12b80 intravenous injections, administered every three weeks (D1, D22, D43). Neutrophil count of 1.500 × 10⁹/L and platelet count of 75 × 10⁹/L are shown (dotted lines). Data are represented as mean ± SD.

Figure 2. 12b80 cardiac toxicity and antitumor activity in dogs.

(A–C) Microscopic photographs of post-mortem cardiac histopathology from three dogs treated with 12b80 cumulative dose of 20 mg/kg (A), 30 mg/kg (B), and 42 mg/kg (C). No dose-limiting cardiotoxicity was noted, rare individual cell necrosis was observed in the dog treated with 42 mg/kg (C, arrowhead), diffuse increased cardiomyocyte cell density was noted in the dog treated with 20 mg/kg (A, ^*) and no histopathological abnormalities were found in the dog treated with 30 mg/kg (B). Original magnification ×40. Hematoxylin-Eosin staining. Bar = 50 μm. (D–I) Microscopic photographs of bone tumor biopsy before treatment (D–F) and post treatment (G–I) in the same three dogs treated with 12b80 cumulative dose of 20 mg/kg (D and G), 30 mg/kg (E and H), and 42 mg/kg (F and I). After treatment, the tumor biopsy shows large necrotic areas with nucleus debris (G–I, arrowheads) or “ghost cells” (G, °). Original magnification ×40. Hematoxylin-Eosin staining. Bar = 50 μm. (J–M) Bone tumor computed tomography in two dogs treated with 12b80 cumulative dose of 24 mg/kg (J, pretreatment; K, post treatment), and 30 mg/kg (L, pretreatment; M, post treatment). Stable disease was noted at day 57 based on the Response Evaluation Criteria in Solid Tumors version 1.0.

Figure 3. Chemical structure of 12b80.

The molecule 12b80 combines doxorubicin bound to a bone targeting hydroxybisphosphonate vector using a pH-sensitive linker. This hydroxybisphosphonate linked doxorubicin compound was designed to specifically trigger doxorubicin release in an acidic bone tumor microenvironment.