A novel drug that
fully eliminated brain tumors from mice in a dramatic 2004 study has shown
a darker side -- causing permanent bone damage in younger mice. The
researcher who conducted both studies says the disappointing new finding
raises concerns about using similar drugs to treat children's cancers, at
least until there is a more thorough understanding of possible risks.
Tom Curran, Ph.D., a developmental biologist at The Children's Hospital
of Philadelphia, led the study, published in the March 2008 issue of the
journal Cancer Cell. The drug in question, HhAntag, is a signal
transduction inhibitor--an agent that blocks signals along a biological
pathway. In mice specially bred for these studies by Curran's research
group, HhAntag specifically acts against signals on a pathway leading to
medulloblastoma, a type of brain tumor found mostly in children.
Much current cancer research has focused on signal transduction
inhibitors (STIs) because of their potential to interrupt specific
biological pathways that give rise to cancer. To date, only one STI has
been approved by the Food and Drug Administration for use in children. That
drug, which acts on different biological pathways than HhAntag does, has
not been associated with any developmental defects in children. However,
other STIs are currently in pediatric clinical trials.
His team's new findings, says Curran, raise a strong caution. "While it
is not clear that the bone defects we observed in mice would also occur in
children, and while signal transduction inhibitors may still represent a
highly promising approach to treating pediatric cancer, it may be important
to perform preclinical testing in young animals before moving ahead to
clinical trials," he added. Young animals could provide a model of a drug's
potential effects during childhood development.
The drug used by Curran's group acts on the hedgehog (Hh) pathway,
which is known to play multiple roles during the development of mammals.
Mutations of genes along that pathway lead to different cancers, including
medulloblastoma, the most common cancerous brain tumor in children. Because
conventional treatment with surgery, radiation and chemotherapy causes
serious long-term side effects such as ataxia (a movement disorder) and
cognitive impairments, the researchers sought novel, less toxic treatments
for medulloblastoma.
In 2001, using genetic engineering, Curran bred mice to develop
medulloblastoma. He then treated those mice with HhAntag, which had
previously been developed by a biotech company for treating skin cancer in
adults. In 2004, while at St. Jude's Children's Research Hospital, Curran
reported highly promising results from the mouse studies. At high doses,
the drug caused the tumors to shrink and in some cases, disappear entirely.
The treated mice also survived much longer than untreated mice, with no
serious side effects.
The drug seemed to be an unusually strong candidate for trials in
children with the type of medulloblastoma having gene mutations on the Hh
pathway--about a third of cases.
However, when Curran's group tested the agent on young mice (10 to 14
days old, in contrast to the adult mice tested previously), they found an
unpleasant surprise: serious impairments to developing bones. The mice were
smaller, with lower weight and shorter bones than untreated mice, and the
effects were not reversible. Even four doses of the drug permanently
stunted their growth. "We already knew that the same biological pathway
involved in the growth of tumors was also involved in bone development,"
said Curran, "but we did not expect temporary inhibition to cause an
irreversible change in bone growth."
While the current studies were disappointing, said Curran, they do not
totally rule out a future role for HhAntag as a treatment for
medulloblastoma. "The effects we see in mice may be less dramatic in
children, and there may be methods of delivering this drug directly to
brain tissue, while avoiding bones. Alternatively, we might discover other
drugs that act on the hedgehog pathway but selectively target brain
tissue." Another approach, he adds, may be to use HhAntag only in older
children who have already completed their growth, or in the admittedly
small proportion of medulloblastoma patients who are adults.
"Signal transduction inhibitors such as this drug may still prove
beneficial in treating children's cancers, but our findings raise questions
about possible adverse effects during childhood development," said Curran.
The National Institutes of Health supported the study. The biotech
company Genentech provided supplies of the drug used in the research.
Curran's co-authors were Hiromichi Kimura, formerly of St. Jude's
Children's Research Hospital; and Jessica M.Y. Ng, of The Children's
Hospital of Philadelphia.
About The Children's Hospital of Philadelphia: The Children's Hospital
of Philadelphia was founded in 1855 as the nation's first pediatric
hospital. Through its long-standing commitment to providing exceptional
patient care, training new generations of pediatric healthcare
professionals and pioneering major research initiatives, Children's
Hospital has fostered many discoveries that have benefited children
worldwide. Its pediatric research program is among the largest in the
country, ranking third in National Institutes of Health funding. In
addition, its unique family-centered care and public service programs have
brought the 430-bed hospital recognition as a leading advocate for children
and adolescents. For more information, visit chop.
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