Dr.
Tad H. Koch
Department of Chemistry & Biochemistry
University of Colorado at Boulder
Professor Koch and his students and co-workers
are investigating aspects of bioorganic chemistry, medicinal chemistry,
photochemistry, and free radical chemistry. They are particularly interested in the chemistry of the
anthracycline quinone antitumor drugs, Adriamycin (doxorubicin), Epirubicin
(epidoxorubicin) and Daunomycin (daunorubicin), relevant to their modes of
biological activity and drug development.
The research group has studied nucleic acid photochemistry and its
application to the elucidation of protein nucleic acid interactions. The
synthesis and properties of amino-carboxy-stabilized carbon radicals such as
3,5,5-trimethyl-2-oxomorpholin-3-yl (TM-3) was an earlier area of
interest. Descriptions of these
projects are expanded as follows.
For questions or suggestions: Tad.Koch@Colorado.EDU
RESEARCH INTERESTS
QUINONE ANITUTMOR
DRUGS
Although
Adriamycin (doxorubicin) and related
anthracycline antitumor drugs are amongst the most important antitumor
drugs in clinical use, their mode of biological activity is not completely
understood. A mechanism proposed more than two decades ago is the alkylation
and cross-linking of DNA via a reductive activation process. Establishing this
mechanism has been difficult because of the instability of DNA-drug adducts and
the lack of a good mechanistic hypothesis. The research group is now developing
a structure based mechanism for the alkylation which first involves sacrificial drug-produced formaldehyde or catalytic drug-produced formaldehyde. Subsequent linking
of the amino group of the drug to the amino group of a guanine base of double
stranded DNA produces a drug-DNA virtual crosslink. The
structure of the virtual crosslink is established by crystallography in 5'-CGCGCG-3' double
stranded oligonucleotide, amongst other techniques. This discovery has led to
the synthesis of dimeric anthracycline-formaldehyde conjugates (Doxoform, Daunoform, and Epidoxoform)
which are active against resistant cancer cells. Activity against resistant
cancer results in part because a resistance mechanism inhibits drug catalyzed
formaldehyde production. Another resistance mechanism keeps the drugs from
their nuclear target. Doxoform, Daunoform and Epidoxoform also overcome this
mechanism as shown in photomicrographs of drug-treated
resistant breast cancer cells. A picture of resistant breast cancer cells
treated with Epidoxoform was featured on the cover of the July 1999 issue of Chemical Research in
Toxicology. Of the three
anthracycline-formaldehyde conjugates, Doxoform has been targeted for further
development because of its exceptional activity against a wide variety cancer
cells. A comparison of the activity of Doxoform with doxorubicin (Dox) against
a panel of 60 human cancer cells by the National Cancer Institute shows a broad
spectrum of activity.
Recent experiments now show the dimeric
doxorubicin-formaldehyde conjugate, Doxoform (DoxF), to be a short-lived
prodrug to the monomeric formaldehyde conjugate, Doxazolidine
(Doxaz). Doxazolidine then
directly cross-links DNA to induce cancer cell death. Doxoform and Doxazolidine show similar activity against a wide variety of cancer cells.
This technology (U.S. patent 6,677,309, Jan. 12, 2004)
and subsequent improvements are available for licensing through the University
of Colorado Office of Technology
Transfer.
Current research focuses on the design, synthesis, and
evaluation of doxorubicin-formaldehyde conjugates targeted to receptors
overexpressed by lung cancer cells and/or immature vascular endothelial cells
associated with the angiogenesis of the respective metastatic lesions. An early design
incorporated an acyclic doxorubicin-formaldehyde conjugate as an N-Mannich base
of salicylamide tethered to a targeting group. The salicylamide served to release the doxorubicin
formaldehyde conjugate with a time constant of about 1 hr. Targets of interest included amino
peptidase N, alpha-v beta-3 integrin, epidermal growth factor tyrosine kinase
domain, estrogen receptor, antiestrogen binding sites, androgen receptor, and
matrix metalloproteinase II. An example targeted to the estrogen receptor and to
antiestrogen binding sites used hydroxytamoxifen as the targeting group. A second example
targeted to the integrin avb3 used a cyclic pentapeptide related to
Celengitide. This example is
featured on the cover of the December 2004 issue of Molecular Cancer Therapeutics.
A limitation of the early design was the lower level
of activity of an acyclic doxorubicin-formaldehyde conjugate relative to that
of the cyclic conjugate, Doxazolidine.
A current design employs Doxazolidine
protected as a carbamate tethered to a self-eliminating spacer with a
carboxylesterase cleavable site.
The construct is thus targeted to cancer cells that overexpress
carboxylesterase enzymes. Analogies include the clinical prodrug of
5-fluorouracil, Capecitabine, and the clinical prodrug of a camptothecin
derivative, Irinotecan, that are substrates for carboxylesterase enzymes.
NUCLEIC ACID PHOTOCHEMISTRY
5-Bromouracil, 5-iodouracil, and 5- iodocytosine are
chromophores readily incorporated into RNA and DNA oligonucleotides
enzymatically or by solid state synthesis. Oligonucleotides bearing these
chromophores are reactive in photocrosslinking
to associated proteins. Multiple photochemical mechanisms are accessible
through careful control of excitation wavelength using laser techniques.
Photocrosslinking of nucleoprotein complexes followed by sequencing establishes
residues in proximity at the nucleoprotein interface. The research group is
presently developing the photocrosslinking technology to select and identify
photo reactive nucleic acids, from a large combinatorial library of nucleic
acids, which will bind and photocrosslink to a target protein and to no other
protein in high yield. The protocol is called PhotoSELEX
and is based upon the SELEX protocol for the selection of high affinity nucleic
acid aptamers from a combinatorial library of nucleic acid sequences. Selected
photo reactive nucleic acids, photoaptamers, may be useful in medical diagnostics to identify a protein
characteristic of a pathological or disease state.
AMINO-CARBOXY-STABILIZED CARBON RADICALS
The 2-oxomorpholin-3-yl radical is an excellent
example of a captodative radical, one that is stabilized by the synergetic
interaction with an electron withdrawing substituent and an electron donating
substituent, the electron withdrawing substituent being the carboxyl
substituent and the electron donating substituent being the amino substituent. The magnitude of the stabilization is of
current interest. TM-3 radical is
formed by dissolving its dimer, TM-3 dimer, at ambient temperature; the radical
simply exists in equilibrium with the dimer. An important property of TM-3 and
related radicals under investigation is their ability to donate a single
electron to a reducible substrate. Some oxomorpholinyl radicals combine reversibly with
molecular oxygen to form peroxides which have potential as photo initiators of
free radical polymerization.
Useful web sites for NMR:
http://www.cis.rit.edu/htbooks/nmr/inside.htm
http://orgchem.colorado.edu/hndbksupport/nmrtheory/NMRtutorial.html
Office: Cristol Chem 159 Laboratory: Cristol Chem 165 & 112
(303) 492-6193 (303) 492-7643
(303) 492-5894 (fax) (303) 492-8095
Web Page revised February 2008.
