Radiopharmaceuticals

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Radiopharmaceuticals

• Lab 7

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• A radiopharmaceutical is a radioactive compound
  used for the diagnosis and therapeutic treatment
  of human diseases.
• In nuclear medicine nearly 95 of the
  radiopharmaceuticals are used for diagnostic
  purposes while the rest are used for therapeutic
  treatment.
• Radiopharmaceuticals usually have minimal
  pharmacologic effect
• In most cases they are used in tracer quantities.

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Ideal Radiopharmaceutical

• Radiopharmaceuticals should possess some
  important characteristics
• Administered to humans
• Several limitations on the detection of
  radiations by currently available instruments

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Easy Availability

• Should be easily produced
• Inexpensive
• Readily available in any nuclear medicine
  facility.
• Complicated methods of production of
  radionuclides or labeled compounds increase the
  cost of the radiopharmaceutical.
• The geographic distance between the user and the
  supplier also limits the availability of
  short-lived radiopharmaceuticals.

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Short Effective Half-Life

• A radionuclide decays with a definite half-life
  which is called the physical half-life Tp (or
  t1/2)
• The physical half-life is independent of any
  physicochemical condition
• Radiopharmaceuticals administered to humans
  disappear from the biological system through
  fecal or urinary excretion, perspiration, or
  other mechanisms

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Short Effective Half-Life

• This biologic disappearance of a
  radiopharmaceutical follows an exponential law
  similar to that of radionuclide decay
• Every radiopharmaceutical has a biologic
  half-life (Tb)
• The net or effective rate (?e) of the loss of
  radioactivity is then related to the physical
  decay constant ?p and the biologic decay constant
  ?b.
• ? e ?p ? ?b
• Te Tp X Tb
• TpTb ?

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Short Effective Half-Life

• The physical half-life of 111In is 67 hr and the
  biologic half-life of 111In-DTPA used for
  measurement of the glomerular filtration rate is
  1.5 hr. What is the effective half-life of
  111In-DTPA?
• 1.47 hr
• Radiopharmaceuticals should have a relatively
  short effective half-life which should not be
  longer than the time necessary to complete the
  study in question

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Particle Emission

• Radionuclides decaying by a- or b-particle
  emission should not be used as the label in
  diagnostic radiopharmaceuticals
• Many b-emitting radionuclides such as
  131I-iodinated compounds are often used for
  clinical studies

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High Target-to-Nontarget Activity Ratio

• For any diagnostic study it is desirable that the
  radiopharmaceutical be localized preferentially
  in the organ under study
• Activity from nontarget areas can obscure the
  structural details of the picture of the target
  organ.
• Target-to-nontarget activity ratio should be
  large.

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Thank You