Epidermal growth factor receptor (EGFR)-associated
protein tyrosine kinase (PTK) complexes have vital anti-apoptotic functions in human
breast cancer cells. We have shown previously that targeting the naturally occurring PTK inhibitor
genistein to the EGFR-associated PTK complexes using the
EGF-Genistein (Gen) conjugate triggers rapid apoptotic cell death in human
breast cancer cells and abrogates their in vitro clonogenic growth. In the present study, we examined the in vivo toxicity profile, pharmacokinetics, and anticancer activity of
EGF-Gen. No toxicities were observed in mice treated with
EGF-Gen at dose levels as high as 40 mg/kg administered i.p. as a single dose or 140 mg/kg administered i.p. over 28 consecutive days.
EGF-Gen significantly improved
tumor-free survival in a
severe combined immune deficiency (SCID) mouse xenograft model of human
breast cancer, when it was administered 24 h after inoculation of
tumor cells. At 100 microg/kg/day x 10 days (1 mg/kg total dose), which is >100-fold less than the highest tested and nontoxic cumulative dose (ie., 140 mg/kg) in mice,
EGF-Gen was more effective than
cyclophosphamide (50 mg/kg/day x 2 days),
Adriamycin (2.5 mg/kg x 1 day), or
methotrexate (0.5 mg/kg x 1 day), the most widely used standard chemotherapeutic drugs for
breast cancer, and resulted in 60% long-term
tumor-free survival. Furthermore, treating SCID mice with established s.c. human
breast cancer xenografts of 0.5-cm diameter with
EGF-Gen at this dose level resulted in disappearance of the
tumors in two of five mice and >50% shrinkage in three of five mice within 10 days, whereas all of the control
tumors in five PBS-treated mice as well as five mice treated with unconjugated Gen (1 mg/kg/day x 10 days) showed >200% increase in diameter during the same observation period.
EGF-Gen treatment reduced the growth rate of
breast cancer xenografts of 1.0-cm diameter, but unlike with
tumors of 0.5-cm diameter, it failed to cause shrinkage or disappearance of these larger
tumors. The level of
EGF-Gen systemic exposure that was effective in SCID mice was achieved in cynomolgus monkeys without any significant side effects detectable by clinical observation, laboratory studies, or histopathological examination of multiple organs.
EGF-Gen might be useful in the treatment of
breast cancer as well as other EGFR-positive
malignancies.