Iodine-129 Tracer Method for Investigating Human Iodine Metabolism
Condition: Healthy Volunteers
Study type: Observational

Summary: 
      Current iodine requirements defined for pregnancy and lactation are rough factorial estimates
      extrapolated from older studies in adults that used radioactive iodine tracers. To ensure
      optimal thyroid function in these vulnerable groups, a tracer method that could be safely
      used to accurately define iodine requirements would be valuable. Iodine-129 (129I), a
      long-lived semi-stable isotope with no health risks, could be used as a tracer, but the
      analytic challenges are formidable. However, we have developed an ICP-MS method to measure
      pp-billion (10-9) to pp-trillion (10-12) quantities of 129I in biological samples. In this
      project we will perform a study in which iodine-replete adult subjects will consume an oral
      dose of 129I. We will quantify 129I kinetic patterns in plasma, urine and stools after the
      oral dose, and use these data to derive tracer absorption, retention and excretion rates.
      This trial will allow us to define optimized procedures for the routine application of this
      method to assess iodine metabolism in humans. The use of 129I may prove to be a breakthrough
      technique to safely assess iodine metabolism and requirements in pregnant/lactating women in
      order to ensure healthy thyroid function in these age groups.
    

Detailed description: 
      Experimental design. The experimental design of this pilot-trial foresees the administration
      of a physiological oral dose of 12 µg of 129I (the tracer) in healthy adults coupled with
      quantitative measurement of the tracer concentration in plasma, urine, and stool samples
      collected prior and following tracer administration. A single dose of 12 µg 129I corresponds
      to a level of activity of 78.5 Bq and an exposure in adults of about 0.16 mSv for the thyroid
      as the target organ. Baseline plasma sample will be collected just before the administration
      of the tracer (-5 min) and follow-up samples will be collected 15, 30, 45, 60, and 90 min, as
      well as 2, 4, 8, 24, 48, 72, and 96 hours following tracer administration. Complete urine
      will be collected over a period of 8 days: baseline samples 1 day before the tracer dose (Day
      3) and follow-up samples 7 days thereafter (until Day 10). Similarly, complete stool
      collection will be done on the day before the tracer dose (Day 3) as well as on the 3
      following days (until Day 6). At Day 20 and 40 (follow-ups), blood sampling and spot urine
      collection will be repeated to verify washed out period. Baseline samples will be used for
      baseline-correction of post-administration samples. In order to minimize variability in the
      response for the purpose of this pilot-trial, the participants will consume a standardized
      diet during the first 6 days of the trial and will keep a food diary during the last 4 days
      of the trial. After suitable preparation, plasma, urine and stool samples will be analyzed
      for tracer quantification (determination of 129I mass on 127I mass; 127I being the naturally
      occurring iodine). From this data, we will construct pharmacokinetic curves of tracer
      appearance and clearance in blood, urine, and stool and we will derive rates and timing of
      human iodine absorption, retention and excretion. In addition, iodine speciation will be
      performed using iodine speciation based on a chromatography method coupled with ICP-MS for
      direct mass quantification of iodine species as plasma inorganic iodine (PII) and
      protein-bound iodine (PBI). This will provide an additional level of information on separate
      pharmacokinetics of metabolized and not metabolized 129I. Because uptake by the thyroid will
      be a determinant of the fraction of the tracer that is retained, the characterization of the
      participants' thyroid function is important to control for potential variation of calculated
      retention in our subjects, and thyroglobulin and thyroid hormones in whole blood will be
      assessed.

      Methods of analysis. The tracer concentration in plasma, urine and stool will be measured by
      a high resolution multi collector ICP-MS with isotope dilution analysis (IDA) using a
      tellurium internal standard following sample digestion according to a method recently
      developed in our laboratory [Dold et al, Thyroid 2015]. For the additional speciation of 129I
      in plasma, analysis will be performed by using ion chromatography ICP-MS analysis (IC-ICP-MS)
      at the Federal Food Safety and Veterinary Office, Swizterland. Urinary iodine concentration
      will be measured by means of the Sandell-Kolthoff reaction [Pino et al, Clin Chem 1996,
      42:239-243] in our laboratory. Thyroglobulin will be assessed using an ELISA dried blood spot
      method recently developed in our laboratory [Stinca et al, Thyroid 2015, 25:1297-1305].
      Thyroid hormones will be measured by the University Hospital in Zurich, Switzerland, using a
      DELFIA (dissociation-enhanced lanthanide fluorescent immunoassay) method. Iodine content in
      the standardized diet will be will be measured in our laboratory by ICP-MS analysis and data
      from the food diaries will be evaluated by using the nutrition software EBIS-Pro (University
      of Hohenheim).

      Data analysis and evaluation. The data generated by this pilot-trial will be used to
      construct pharmacokinetic curves of tracer appearance and clearance in the collected samples
      (total plasma, plasma PII-fraction, plasma PBI-fraction, urine, stools). Curves will be
      constructed by fitting a LOESS multiple regression model, a non-parametric locally weighted
      scatter plot smoothing of tracer concentration in samples as a function of time, and provide
      uncertainty range to visually represent the tracer's pharmacokinetic patterns and variability
      of response. For the evaluation of tracer kinetic patterns we will generate classic
      pharmacokinetic modelling parameters, including maximum tracer concentration (cmax), time at
      maximum tracer concentration (Tmax), elimination rate constant (ke), absorption rate constant
      (ka), and area under the curve (AUC), by using the simulation analysis and modelling software
      SAAM II (University of Washington). Total tracer retention will be calculated as total tracer
      intake - total tracer excreted (AUCurine + AUCstool). Total tracer absorption will be
      calculated as total tracer intake - total tracer appearance as plasma inorganic iodine
      (AUCPII). The optimization of procedures for routine application of this method will mainly
      include the definition of minimal requirements of specimen's specific sampling duration
      (number of days) after the administration of the tracer dose. This will be assessed by using
      ROC curves.

      Expected output and significance/ expected impact. This study will be the first to use 129I
      as a biologic tracer in humans. Once developed, the availability of this novel method would
      provide a unique tool to address several key issues with regards to human iodine metabolism
      and thyroid function. These include the precise definition of the iodine requirement for
      important life stages, investigation of potential up-regulation of iodine absorption during
      iodine deficiency, thyroid uptake and turnover of iodine in varying iodine status, assessment
      of placental and breast milk iodine transfer, as well as the assessment of iodine
      bioavailability from different foods. Thus, this project could have significant long term
      impact in the field of iodine and the thyroid.
    

Intervention name(s): Iodine-129
Intervention type(s): Other
Intervention description(s): The present study is an observational study and does not therefore include an investigational product in the proper sense because the used tracer (semi-stable iodine isotope) has no physiological effect on the participants. 129I will be used as tracer to measure appearance in plasma, urine and stools after the oral administered of the 129I-tracer.
Other intervention name(s): 129I
Observational model: Other
Time perspective: Prospective
Sampling method: Non-Probability Sample
Study population: 
        Generally healthy, iodine-replete males and non-pregnant, non-lactating females, who are
        between 18 and 45 years old, whose body weight is less than 80 kg and BMI between 19-27.5
        kg/m2, who regularly consume iodized salt, who have signed the informed consent, and who
        have good knowledge of the English language. They will serve as an iodine-sufficient
        population with normal iodine metabolism.