Whole-Body Biodistribution Imaging in NHPs | 125I SPECT

Understanding where a therapeutic distributes in the body is part of preclinical drug development. Plasma pharmacokinetics can describe systemic exposure, but they say nothing on their own about how a compound spreads across tissues, or how tissue-associated exposure changes over time.

Traditional tissue distribution studies answer that question by terminal sampling and ex vivo analysis of harvested organs. Molecular imaging offers a complementary route: incorporate a suitable radionuclide into the test article, then visualize and quantify tracer-associated distribution in vivo, and repeat the measurement in the same animal at multiple time points.

A case from the Prisys Biotechnologies webinar Molecular Imaging in NHP Models for Translational Drug Development illustrates the approach with an intravenously administered ¹²⁵I-labeled test article in non-human primates (NHPs). The study was designed to follow whole-body distribution over time and to derive organ-level measurements from the imaging data.

The case shows how molecular imaging adds a longitudinal and spatial dimension to conventional biodistribution work, while leaving the option of ex vivo confirmation open for studies that need it.

Drug exposure is not uniform throughout the body. Distribution into individual organs and tissues affects pharmacological activity, target engagement, off-target exposure, and safety.

That matters most for modalities whose biological activity depends on tissue localization. Biologics, peptides, antibodies, antibody-based therapeutics, cell therapies, and radiolabeled compounds often require an understanding of how the administered material moves from the circulation into target and non-target tissues.

Conventional plasma PK describes systemic exposure through parameters such as Cmax, AUC, and clearance. Plasma concentration, however, does not predict tissue exposure. Two compounds with similar plasma PK profiles may distribute very differently.

Biodistribution studies supply the missing layer by answering a different question: where does the administered material go, and how does its tissue-associated signal change over time?

A conventional tissue distribution study uses predefined terminal time points. Animals are euthanized at each point, organs are collected, and the amount of test article or radiolabeled material is quantified ex vivo.

The method remains established and valuable. It gives direct measurements from harvested tissues, and it supports downstream work that imaging alone cannot provide, including detailed biochemical, histopathological, or cellular characterization.

Its limitation for longitudinal work is built into the design. Each animal generally contributes tissue data at one terminal time point, so a time course requires different animals at different sampling points.

Molecular imaging changes the temporal structure of the experiment. With an appropriately radiolabeled test article, the same subject can be imaged repeatedly, producing a sequence of observations in place of independent terminal measurements.

That within-subject design yields information on the spatial distribution of the tracer, organ-associated accumulation, and signal change over time. The Prisys webinar case illustrates the principle with whole-body ¹²⁵I imaging in NHPs, describing serial imaging across multiple time points, organ-level quantification, and a microdose study design.

The workflow has four stages: choosing a radionuclide, radiolabeling the test article, in vivo imaging, and quantitative analysis of the image data.

Radionuclide selection follows from the molecular characteristics of the test article, the expected biological kinetics, and the imaging modality. The molecular imaging platform presented in the webinar supports both PET and SPECT radionuclides, including ¹⁸F, ⁶⁸Ga, ⁶⁴Cu, ⁸⁹Zr, ¹²⁴I, ¹¹C, ¹³¹I, ¹²⁵I, and ^99mTc. The presentation lists ¹²⁵I with a physical half-life of approximately 59.4 days and names peptides, proteins, and antibodies among the applicable labeled compounds.

For longer-lived radionuclides such as ¹²⁵I, SPECT/CT can visualize the distribution of radiolabeled material over an extended observation window. PET radionuclides such as ¹⁸F, ⁶⁸Ga, ⁶⁴Cu, and ⁸⁹Zr offer different combinations of half-life, sensitivity, and imaging characteristics, and may suit shorter or intermediate biological time scales.

Producing an image is not the endpoint. Quantitative molecular imaging depends on calibration, acquisition protocols, reconstruction, region-of-interest or organ segmentation, and conversion of measured activity into a biologically interpretable metric.

One metric used widely in biodistribution research is percentage injected dose per gram of tissue (%ID/g). It expresses tissue-associated radioactivity relative to the administered activity and normalizes to tissue mass. Where imaging data are appropriately calibrated and organ volumes or masses can be estimated, %ID/g gives a quantitative framework for comparing tracer-associated distribution across organs and time points.

Accuracy of image-derived quantitation depends on several things at once: the radionuclide, the imaging system, the acquisition and reconstruction parameters, attenuation and scatter correction, organ segmentation, calibration procedures, and the study conditions. Measurements derived from images should therefore be interpreted within the validation framework of the individual study.

The webinar case used an intravenously administered ¹²⁵I-labeled test article to investigate whole-body tissue distribution in NHPs.

Instead of collecting tissues at a single terminal time point, imaging was performed at multiple time points to follow the radiolabeled material longitudinally. The study produced spatial information on whole-body distribution while allowing repeated observation of the same animals.

Imaging data were used to evaluate organ-associated tracer signal and to calculate organ-level drug content from the measured uptake. The case illustrates distribution across major organs and shows how molecular imaging supplies both spatial and temporal information that conventional terminal sampling alone does not easily provide.

Note that this was presented as an application example of molecular imaging, not as a replacement for conventional tissue analysis. Its value lies in adding a longitudinal in vivo layer to the biodistribution dataset.

¹²⁵I suits biological questions that extend past a short imaging window. Its relatively long physical half-life allows radiolabeled material to be followed over extended periods; the appropriate imaging schedule still depends on the pharmacokinetics of the test article and on the labeling strategy.

In vivo molecular imaging adds more than a picture of drug distribution. Its scientific value lies in combining spatial information with repeated measurement.

First, longitudinal imaging follows changes in tissue-associated signal within the same subject, which reduces the contribution of inter-animal variability when temporal changes are evaluated.

Second, whole-body imaging provides spatial context. Instead of restricting analysis to organs selected for terminal collection, the imaging field covers a broader view of where the radiolabeled material goes.

Third, quantitative image analysis generates organ-level measurements that complement plasma PK and conventional tissue concentration data. This helps when investigators need to connect systemic exposure with tissue-associated exposure.

Finally, imaging can inform the design of subsequent terminal studies. Longitudinal imaging may identify organs or time points that warrant more detailed ex vivo characterization, so terminal sampling can be built around specific biological questions rather than predefined assumptions.

These uses match the broader role of molecular imaging in preclinical development described in the webinar, where imaging covers biodistribution, pharmacokinetics, target engagement, pharmacodynamics, efficacy assessment, and translational dose evaluation.

Molecular imaging is not a universal replacement for terminal biodistribution analysis.

Image-derived measurements represent activity within a defined imaging volume, and they carry the spatial resolution and quantitative limitations of the imaging system. Imaging also has limited ability to distinguish extracellular, cellular, or subcellular localization of the labeled material.

Ex vivo analysis provides direct tissue measurements and can be combined with histopathology, autoradiography, bioanalysis, or other tissue-level assays. These methods remain necessary when definitive tissue concentration or mechanistic characterization is required.

In practice the two are used sequentially or together. Longitudinal molecular imaging characterizes whole-body distribution and flags the organs and time points that matter; a targeted terminal cohort then supplies direct tissue measurements and deeper mechanistic characterization.

An integrated design of this kind extracts more information from each animal while preserving the analytical strengths of ex vivo methods.

Radionuclide selection should follow the biological question, not the imaging modality alone.

Short-lived PET radionuclides such as ¹⁸F suit rapid pharmacokinetic or receptor-binding processes. Longer-lived PET radionuclides such as ⁸⁹Zr can be useful for slowly distributing biological therapeutics, while SPECT radionuclides such as ¹²⁵I provide extended observation windows for appropriately labeled compounds.

The platform described in the webinar covers the full PET and SPECT panel listed above. The presentation also stresses that isotope selection belongs in the same discussion as the characteristics of the compound and the intended application.

For NHP studies, additional considerations include labeling stability, preservation of the biological activity of the parent molecule, specific activity, radiochemical purity, expected tissue kinetics, radiation exposure, imaging sensitivity, and the feasibility of repeated anesthesia and imaging.

Quantitative molecular imaging connects preclinical exposure with translational decision-making.

The webinar presents whole-body biodistribution imaging as a source of information for target-tissue exposure assessment, PK modeling, allometric considerations, and first-in-human dose selection.

Treat these applications as part of an integrated translational dataset, not as standalone determinants of clinical dose. Image-derived exposure information is most informative alongside conventional PK, pharmacology, target engagement, safety, and other nonclinical evidence.

The translational value comes down to connecting three dimensions of exposure: systemic exposure measured in blood, spatial exposure observed across tissues, and temporal change in tissue-associated signal.

Prisys Biotechnologies incorporates a biodistribution imaging service into its NHP translational research platform, where it supports studies in biodistribution, CNS exposure, target engagement, disease progression, and pharmacodynamic assessment.

The platform combines radiolabeling capabilities with large-animal PET/CT and SPECT/CT imaging. The webinar materials describe PET/SPECT tracer production and labeling for small molecules, peptides, proteins, macromolecules, nanomaterials, and antibodies, alongside large-animal multimodal imaging capabilities.

Within NHP studies, molecular imaging can be integrated with conventional pharmacology and PK/PD assessments, adding complementary spatial information without making every longitudinal observation depend on terminal tissue collection.

The wider Prisys imaging platform includes MRI, CT, DSA, and other clinical-equivalent imaging modalities, which supports longitudinal assessment across multiple therapeutic areas.

Whole-body molecular imaging gives a quantitative, longitudinal read on drug biodistribution in non-human primates. The ¹²⁵I case in the Prisys Beyond Anatomy webinar shows how serial SPECT imaging follows radiolabeled drug distribution across the body and generates organ-level information from the same animals.

It does not remove the need for conventional ex vivo analysis. It adds a complementary layer of spatial and temporal data that strengthens how biodistribution studies are interpreted and designed.

For translational drug development, combining longitudinal imaging with systemic PK, tissue analysis, and pharmacodynamic endpoints gives a more integrated view of drug exposure from administration to target tissue.

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