Uses Of Radiopharmacology

Radiopharmacology
This is what really drew me in to the field. Relatively speaking, there are really only a small number of radiopharmaceuticals and nuclear isotopes used, and this allows us to become experts on a good majority of them. 80% of radiopharmacy is diagnostic; however, there are some fascinating and effective therapeutic drugs that we compound as well. Now, I’m not going to go into each and every drug, but these are the big ones, and it’ll give you a good taste for what’s out there.

• Cardiology: this is the bread and butter of nuke med and the major agents used are Thallium-201, Tc-99m Sestamibi (Cardiolite®) and Tc-99m Tetrofosmin (Myoview™). They’re useful in myocardial perfusion imaging (i.e. comparing a ‘rest’ and ‘stress’ image to identify ischemia/infarction), avid infarct imaging (to detect damaged myocardial tissue post-MI) and cardiac function studies (to determine how well the heart is pumping via LVEF). These studies are a great tool for guiding a patient’s course of therapy...determining whether they may need open heart surgery, cath, or strictly risk management with lipid control, etc.

Brain imaging: Exametazime (Ceretec™), Bicisate (Neurolite®); these agents can be used to screen for tumors, detect metastases, detect intracranial injury, determine legally defined ‘brain death’, identify seizure foci, etc.

• Skeletal imaging: Tc-99m Medronate (MDP) and Tc-99m Oxidronate (HDP); are radiotracers with a bisphosphonate structure that you’re all familiar with. These can be used to assess trauma (i.e. fracture imaging), distinguishing osteomyelitis vs cellulitis, evaluate bone cancer/multiple myeloma, paget’s disease, etc.

Treatment of bone metastasis: here’s a good example of where nuke medicine is used for treatment rather than strictly for diagnosis. Sr-89 Chloride (Metastron®) and Sm-153 Lexidronam (Quadramet®) can be far more effective than traditional therapy in helping cancer patients suffering from excruciating pain from bone mets.

Liver/Spleen imaging: Tc-99m Sulfur Colloid; used to image for hepatitis/cirrhosis, high LFT’s, liver tumor, trauma, abscesses, etc.; where ‘cold spots’ (dark areas) will indicate an abnormality.

Lymphoscintigraphy: small doses of Tc-99m Sulfur Colloid are injected during surgery to locate lymphatic drainage patterns, guide oncological surgeons, and to identify the location of a sentinel node. The sentinel node (first node downstream from the tumor) can then be sent for biopsy to determine metastasic status.

Hepatobiliary imaging: Tc-99m Mebrofenin (Choletec®) is used for gallbladder imaging to differentiate between acute (oftentimes caused by gallstones) and chronic cholecystitis. In acute cholecystitis, the gallbladder will light up in the scan, but does not for chronic disease.

Renal imaging: Tc-99m Pentetate (DTPA) and Tc-99m Mertiatide (MAG-3) are used for renal function imaging (i.e. quantifying GFR or tubular secretion), whereas Tc-99m Succimer (DMSA) is used to assess structure/anatomy of the kidney. These are useful in patients with renal obstruction, renal HTN, tumor, trauma, etc.

Pulmonary imaging: VQ scans are done to differentiate between a pulmonary embolism (lung clot) and COPD. A perfusion test (using Tc-99m MAA) is done first. If the results are abnormal, the ventilation portion of the study (using radioactive Xe-133 gas or aerosolized Tc-99m DTPA) is performed. Normal ventilation will then indicate that the patient has a PE, whereas abnormal ventilation points to COPD.

• Thyroid imaging and treatment: since the thyroid gland naturally takes up iodine in order to produce thyroid hormones, administering radioactive iodine is a logical step in order to assess function (uptake) of the thyroid, as well as image or treat thyroid cancer. Thyroid uptake/function studies are performed by administering I-123 or I-131 NaI, helping to determine hypo-/hyperthyroidism. Thyroid imaging can also be performed to assess ‘hot’ or ‘cold’ nodules on the thyroid; as well as whole body imaging, to look for metastatic tumors during follow-up of thyroid cancer. Thyroid therapy is a classic example of how nuclear medicine is used for treatment purposes. I-131 NaI is administered in higher activities to treat hyperthyroidism (treatment of choice), as well as ablate the gland after surgery to mop up any remaining cells.

• Infection imaging: Ga-67, which is similar to iron, is passively localized to a site of infection and is the drug of choice for chronic infection imaging. Radiolabeled white blood cells can be used to image acute infection, inflammatory bowel disease, fever of unknown origin, osteomyelitis, soft tissue abscess, skin graft infection, diabetic foot ulcer, etc. A hospital will send us a syringe containing 60 cc of the patient’s blood. In our blood room (a completely segregated area from the remainder of the pharmacy), we use a completely needless procedure, involving centrifuging the sample, to extract the patient’s white blood cells. The WBC’s are then tagged with radioactive In-111 or Tc-99m Exametazime (Ceretec™). The patient’s own radio-labeled WBC’s are then sent back to the hospital, within 5 hours they are re-injected into the patient, and scanned to localize the site of infection. On a busy day, we’ll do anywhere up to 12 - 15 bloods.

• Monoclonal antibody imaging/therapy: this could quite possibly be the future for nuclear medicine, as there are so many possible applications for monoclonals. There are a only a handful of agents available now: In-111 Capromab Pendetide (ProstaScint®) to image prostate cancer, and In-111/Y-90 Ibritumomab Tiuxetan (Zevalin™) or I-131 Tositumomab (Bexxar®); however many more are in production. Zevalin™ and Bexxar® are effective treatment options for patients with non-Hodgkin’s lymphoma.
  

• PET: this is actually another fascinating area of nuclear medicine that I could go into a lot of detail, but to keep it simple…FDG (think radioactive glucose), which is produced at a facility with a cyclotron, is used to detect areas of the body undergoing high metabolism (think epilepsy, cancer) relative to normal tissue. Since PET looks at the disease on a chemical level, you can identify the disease much sooner than when using other imaging modalities.

 

• Adjunctive agents: there are a handful of non-radioactive meds that we dispense a lot of too, and they include pharmacological stress agents which are used when a patient isn’t able to exercise prior to a ‘stress’ portion of a myocardial study (i.e. dipyridamole, adenosine, regadenoson, dobutamine); aminophylline (an antidote used to reverse stress agents); and sincalide (to cause the gallbladder to empty for hepatobiliary imaging).