Richard Ting

Toward [18F]-labeled aryltrifluoroborate radiotracers: in vivo positron emission tomography imaging of stable aryltrifluoroborate clearance in mice.

The use of a boronic ester as a captor of aqueous [(18)F]-fluoride has been previously suggested as a means of labeling biomolecules in one step for positron emission tomography (PET) imaging. For this approach to be seriously considered, the [(18)F]-labeled trifluoroborate should be humorally stable such that it neither leaches free [(18)F]-fluoride to the bone nor accumulates therein. Herein, we have synthesized a biotinylated boronic ester that is converted to the corresponding trifluoroborate salt in the presence of aqueous [(18)F]-fluoride. In keeping with its in vitro aqueous kinetic stability at pH 7.5, the trifluoroborate appears to clear in vivo quite rapidly to the bladder as the stable trifluoroborate salt with no detectable leaching of free [(18)F]-fluoride to the bone. When this labeled biotin is preincubated with avidin, the pharmacokinetic clearance of the resulting complex is visibly altered. This work validates initial claims that boronic esters are potentially useful as readily labeled precursors to [(18)F]-PET reagents.

Substituent Effects on Aryltrifluoroborate Solvolysis in Water: Implications for Suzuki−Miyaura Coupling and the Design of Stable 18F-Labeled Aryltrifluoroborates for Use in PET Imaging

Whereas electron withdrawing substituents retard the rate of aryltrifluoroborate solvolysis, electron-donating groups enhance it. Herein is presented a Hammett analysis of the solvolytic lability of aryltrifluoroborates where log(k(solv)) values correlate to sigma values with a rho value of approximately -1. This work provides a predictable rubric for tuning the reactivity of boron for several uses including (18)F-labeled PET reagents and has mechanistic implications for ArBF(3)-enhanced ligandless metal-mediated cross coupling reactions with aryltrifluoroborates.

Novel Matrix Metalloproteinase Inhibitor [18F]Marimastat-Aryltrifluoroborate as a Probe for In vivo Positron Emission Tomography Imaging in Cancer

Matrix metalloproteinases (MMP), strongly associated pathogenic markers of cancer, have undergone extensive drug development programs. Marimastat, a noncovalent MMP inhibitor, was conjugated with FITC to label cellular metalloproteinase cancer targets in MDA-MB-231 cells in vitro. Punctate localization of active transmembrane MMP14 was observed. For molecular-targeted positron emission tomography imaging of syngeneic 67NR murine mammary carcinoma in vivo, marimastat was (18)F-labeled using a shelf-stable arylboronic ester conjugate as a captor for aqueous [(18)F]fluoride in a novel, rapid one-step reaction at ambient temperature. [(18)F]Marimastat-aryltrifluoroborate localized to the tumors, with labeling being blocked in control animals first loaded with >10-fold excess unlabeled marimastat. The labeled drug cleared primarily via the hepatobiliary and gastrointestinal tract, with multiple animals imaged in independent experiments, confirming the ease of this new labeling strategy.

Towards kit-like 18F-labeling of marimastat, a noncovalent inhibitor drug for in vivo PET imaging cancer associated matrix metalloproteases

Marimastat, a clinically trialed drug developed to treat breast cancer by inhibiting cancer-associated matrix metalloproteases (MMPs), was linked to an aryl boronic ester for single-step [18F]-aqueous fluoride capture and the labeled product revealed tumor associated MMP activity in vivo. Herein, we report important radiosynthetic attributes for labeling marimastat that enabled the first PET images of breast cancer-associated matrix metalloproteases in a syngenic murine model. The advantages of this method include one-step post synthetic labeling in less than one hour at ambient temperature, the ability to work in aqueous media without drying the 18F-fluoride, observation of high radiochemical purity, and the potential for tripling the specific activity of the fluoride used in labeling. Using low levels of activity e.g. 60 mCi in low volumes this method affords reasonable yields of labeled marimastat with decay-corrected specific activities of 0.39 and 0.75 Ci/μmol, and real specific activities of 0.16 and 0.39 Ci/μmol. Current limitations of this method along with anticipated improvements are discussed.

From Minutes to Years: Predicting Organotrifluoroborate Solvolysis Rates

Organotrifluoroborates solvolyze in water at rates that vary over five orders of magnitude. The negative logarithm of the solvolytic rate constant, pk(B-F), correlates exceptionally well with the pKa of the analogous carboxylic acid (R(2) = 0.984). This unforeseen correlation may be of predictive value for several applications including Suzuki-Miyaura cross-coupling reactions and the design of (18)F-organotrifluoroborate radioprosthetic groups.

Selection and characterization of DNAzymes with synthetically appended functionalities: A case of a synthetic RNAsea mimic

We have been interested in merging synthetic nucleotide chemistry with combinatorial selection of DNAzymes to deliver a more complete (and complex) chemical complement to the catalytic repertoire of nucleic acids. Thus we ask, what do modified dNTPs really bring to nucleic acids in terms of an increased repertoire? In asking this question, we have looked first at conditions, and more recently for reaction classes where nucleic acids are found to be catalytically inefficient, deficient, or at least to date, seemingly incapable of certain functions. A case of this is M2+-independent ribophosphodiester hydrolysis at physiological pH and low ionic strength where nucleic acids exhibit especially low rate constants for self-cleavage and seem to be incapable of turnover.