Preparation and Evaluation of ^99mTc-labeled Tridentate Chelates for Pre-targeting Using Bioorthogonal Chemistry

The overall goal of this procedure is to produce technetium-99m labeled tetrazines for pre-targeted imaging using bioorthogonal chemistry with trans-cyclooctene modified biomolecules as targeting vectors. This method provides an important new platform for the development of novel radio pharmaceuticals. Through pre-targeting the technetium complexes described here can be used to image a wide array of organs, tissues, and bio-markers.

This technique is advantageous as it provides a robust step-by-step method to produce tetrazines radio-labeled with technetium-99m, the most widely used medical isotope in diagnostic medicine. This technique can be used with different targeting vectors, including small molecules and antibodies. It can also be used for radio therapy by exchanging technetium with beta-emitting isotopes of rhenium.

Demonstrating part of the animal procedures will be Nancy Janzen, biology technician in our lab. To begin the radio-labeling procedure, in a microwave vial, combine eight milligrams of potassium boranocabonate, 15 milligrams of sodium carbonate, 20 milligrams of sodium tetraborate decahydrate, and 25 milligrams of potassium sodium tartrate tetrahydrate. Purge the vial with argon gas for 10 minutes.

Then, add four milliliters of technetium-99m pertechnetate in 0.9%saline with an activity of 1, 110 megabecquerels to the vial. Load the reaction mixture into a microwave reactor, and stir for 10 seconds. Then, heat the mixture at 110 degrees Celsius for 3.5 minutes to obtain the triaqua tricarbonyl technetium-99m cation.

Add about 400 microliters of one molar hydrochloric acid to adjust the solution pH to 3.5 to four. Next, for each tetrazine ligand, dissolve two milligrams of ligand in 250 microliters of methanol. Add 250 microliters of the technetium-99m complex with an activity of 74 megabecquerels to each ligand solution.

Heat mixture in a microwave reactor at 60 degrees Celsius for 20 minutes. For tetrazine ligands with t-Butyl protecting groups, transfer the solution from a microwave vial to a 20-milliliter scintillation vial, and then, evaporate the solvent and redissolve each complex in one milliliter of a one-to-one by volume solution of dichloromethane and trifluoroacetic acid. Then, transfer the solution to a microwave vial, and heat those complexes in a microwave reactor at 60 degrees Celsius for six to 10 minutes to de-protect the ligands.

Cool the complexes to room temperature, and then, evaporate the solvent. Redissolve the residues in 500 microliters of a one-to-one aqueous solution of either acetonitrile or methanol and water before proceeding to HPLC purification. Use C18 reversed phase HPLC to purify each radio-labeled tetrazine complex with an elution gradient of 30 to 70 to 40 to 60 acetonitrile to water over 20 minutes with 0.1%trifluoroacetic acid present in the eluent throughout.

Collect the purified product when shown eluding in the gamma trace. Then, co-inject each labeled complex with a solution of 0.125 milligrams of the corresponding radium-tetrazine complex in 20%methanol in water. Compare the retention times of the rhenium complex peak in the UV trace and the technetium complex peak in the gamma trace to verify the identity of the gamma peak.

Remove the solvent from each purified technetium-tetrazine complex. Formulate each complex to a concentration of 7.4 kilobecquerels per microliter in phosphate-buffered saline with 0.5%bovine serum albumin and 0.01%polysorbate 80. For each mouse subject, draw into a syringe 100 microliters of a five microgram per microliter solution of the TCOPB derivative in saline to prepare a 20 milligram per kilogram dose.

Connect a 30-gauge needle to the dosing syringe. Place the mouse subject in a physical restraint device. Identify the veins on the lateral surface of the tail approximately two centimeters from the end of the tail, and wipe the area with an alcohol swab.

Insert the syringe needle at a shallow angle parallel to the vein. Slowly inject the TCOBP derivative. Remove the needle and apply a clean gauze sponge with slight pressure to the injection site until bleeding stops.

One hour after injection of the TCOBP derivative, administer a 100-microliter dose of the desired technetium-99m tetrazine complex with an activity of approximately 0.74 megabecquerel by tail vein injection. Use a dose calibrator to measure the activity of a 100-microliter test sample of the technetium complex at time of injection. Save the test sample for later measurements.

Anesthetize the mouse with a 3%isoflurane and 2%oxygen gas mixture six hours after injection. Place the mouse on its back with its nose in the nose cone for continued anesthesia, and locate the xiphoid process. Connect a 25-gauge needle to a syringe pre-treated with heparin.

Insert the needle at a 20 degree angle under the xiphoid process, slightly to the left of the midline. Once the needle is fully inserted, slowly withdraw the plunger to check for blood in the needle hub, confirming a successful heart puncture. Readjust the needle as needed.

Once the heart has been punctured, slowly draw one milliliter of blood into the syringe. Transfer the blood sample to a pre-weighed counting tube. Euthanize the mouse by cervical dislocation while under anesthesia.

Place the euthanized animal in a plastic bag and measure the whole body activity with a dose calibrator. Then, collect the knee and shoulder bones, the gallbladder, the kidneys, the liver, the thyroid and trachea, and the tail. Along with their contents, collect the stomach, the small intestines, the large intestines and cecum, and the urinary bladder.

Rinse the blood from all collected tissues except the gall and urinary bladders in phosphate-buffered saline. Blot dry the tissues, and place each tissue in a pre-weighed counting tube. Measure the residual activity of the carcass.

Weigh each tissue sample, and calculate the mass of the tissue alone. Use a multi-detector gamma counter to measure the activity of a five-microliter aliquot of the technetium complex test sample. Using the obtained value in counts per minute and the activity measured at time of injection, calculate a conversion factor.

Measure the activity of each tissue and fluid sample with the gamma detector. Convert the values to microcurie, and normalize each value by organ weight to determine the percent injected does per gram of tissue. Repeat this process with additional mice without the injection of the TCOPB derivative as a negative control series.

Using this method, five tetrazine complexes differing in their linker and key-leader combinations were labeled with technetium-99m. A TCOPB derivative, which localizes to regions of high calcium turnover in bone, was used to pre-target the shoulder and knee joints of mice. The ratios of percent injected dose per gram of tissue for bone to blood were calculated for each tetrazine compound.

All five compounds cleared the blood rapidly and targeted bone, indicating that the selective bioorthogonal coupling reaction between the tetrazine compounds and the TCOBP derivative had occurred in vivo. Compound three showed the best combination of targeting and clearance from blood. Negative control experiments indicated that the radio-labeled tetrazine complexes did not significantly localize to bone in the absence of the TCOBP derivative.

For instance, when the TCOBP derivative was not administered, compound two had high levels in the gallbladder and intestines but very little in bone. Wide variation was observed in tissue localization in mice receiving the TCOBP derivative among the five compounds tested. For example, very little of compound three was seen in the large intestine, liver, or urine and bladder compared to other complexes, whereas much more of compound three was seen in bone.

Once mastered, the labeling technique described here can be performed from start to finish in less than two hours if performed correctly. While attempting this technique, it's important to ensure all reagents are weighed out accurately and that the salts are purged with argon before addition to the technetium solution. It is also critical that the pH of the technetium solution is adjusted to 3.5 to four before addition of the tetrazine ligand.

After watching this video, you should have a good understanding on how to prepare and purify technetium-labeled tetrazines and how they can be used to create new radio pharmaceuticals using pre-targeting and bioorthogonal chemistry. Don't forget to take appropriate precautions when working with radioactivity, such as understanding the type of radiation emitted and how to best shield and handle samples.