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Molecular Therapy — Nucleic AcidsPharmacology & ToxicologyPeer-reviewed journal article

Biodistribution of mRNA vaccines in rats: Enrichment in injection site and lymph tissues and rapid clearance without tissue persistence

Susan M.G. GoodyiD, Christopher RowbottomiD, Yun Liu, Nancy Chen

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Abstract

Messenger RNA (mRNA) vaccines using lipid nanoparticles (LNPs) are well-established and globally approved with acceptable safety profiles for preventing respiratory disease. Other mRNA-LNP product concepts are also emerging as novel treatments for broader clinical use. Here, we describe mRNA-LNP vaccine tissue distribution and kinetics after intramuscular dosing using three products formulated with same LNP matrix: mRNA-1273 (Spikevax), mRNA-1647 (a candidate cytomegalovirus [CMV] vaccine), and a reporter mRNA (nascent peptide-luciferase) drug product. Consistent biodistribution patterns were observed across studies: tissues with highest exposures were the injection site, draining lymph nodes, and spleen, with minimal distribution to non-lymphoid tissues. Vaccine components cleared rapidly from circulation and tissues, with complete elimination simulated to occur by [~]2 weeks. Following mRNA-1273 vaccination, Spike protein levels were transiently observed (elimination <5 days) and did not accumulate with repeated dosing. The ionizable lipid in the LNP matrix, Lipid H, underwent biotransformation and was excreted renally and hepatically, with no human-specific metabolites. Collectively, these results indicate that the LNP composition, not mRNA cargo, governs biodistribution. Furthermore, in a SARS-CoV-2 infection-free model, there was no evidence of Spike protein persistence. Overall, the data establish a framework that justifies leveraging biodistribution data across products and supports eliminating redundant animal studies.

In-depth summary

Claude Sonnet 4.6 · xhigh effort · 8 Nov 2026 · from the full text of version 1 (25 Jan 2026) · CC BY 4.0

Three mRNA-LNP products sharing one lipid matrix show rat intramuscular biodistribution (injection site, lymph nodes, spleen) and rapid clearance, with no Spike accumulation after repeat dosing.

Biodistribution datasets for licensed mRNA-LNP vaccines sit in proprietary regulatory dossiers, and human studies cannot exclude prior or subclinical SARS-CoV-2 infection when assessing Spike persistence. This work reports harmonized cross-product rat data plus Spike kinetics in an infection-naive model.

Key results

  1. ShownAcross mRNA-1273, mRNA-1647 and the NPI-Luc reporter, exposure of mRNA and Lipid H was highest at the injection site, draining lymph nodes and spleen. Distal tissues were minimal, with liver the highest among them.

    Tissue-to-systemic AUC ratios were >100% for lymphoid tissues and <100% for distal tissues. Liver was ~3% of injection-site exposure; bone marrow, brain, heart, lung, testes and ovaries were <1% (Tables 1 and 2). · Across drug products, tissues with the highest…; Discussion

  2. SuggestedSerum mRNA and Lipid H declined rapidly, with half-lives of a few hours. Lymphoid-tissue mRNA half-lives were longer but still short. First-order simulation projected near-complete mRNA elimination by about 2 weeks.

    Serum T1/2 ~3 h (mRNA) and ~8 h (Lipid H); mRNA T1/2 24 h to 48 h in lymph nodes and ~63 h in spleen. Simulation over six half-lives left ~1.6% of Cmax (>98% eliminated) at ~2 weeks (Table 5). · No persistence of mRNA, Lipid H, or SARS-CoV-2…; Simulation of mRNA and Spike protein elimination…

  3. SuggestedAfter mRNA-1273, serum Spike protein was transient. Among distal tissues only liver had measurable Spike, and it sat near the LLOQ. Spike was undetectable in brain, heart and lung.

    Spike peaked at 24 h with effective serum T1/2 of 18 h. Quantifiable serum levels lasted up to 3 days after dose 1 and up to 7 days after dose 2. Simulated elimination was >98% within 5 days (Table 6). · No persistence of mRNA, Lipid H, or SARS-CoV-2…; Discussion

  4. ShownA second mRNA-1273 dose on Day 28 did not increase mRNA or Lipid H exposure despite a strong anti-Spike S2P antibody response. This argues against accumulation and against an immune-response effect on distribution.

    Cmax and AUC in serum, injection site, axillary lymph nodes and spleen showed no significant Day 1 vs Day 28 differences (unpaired t-test, 4 animals/sex/timepoint/dosing occasion). Baseline antibody titers were <30 AU/mL, then rose by orders of magnitude. · Lack of accumulation of mRNA, Lipid H and Spike…; In vivo biodistribution studies; Statistical analysis

  5. ShownLipid H was cleared mainly by biotransformation and excreted through urine and bile. Hepatocytes from human, rat and monkey showed conserved pathways with no human-specific metabolites.

    12 metabolites were identified (ester hydrolysis, β-oxidation, hydroxylation, glutathione conjugation). Urine was dominated by M1 to M4 within 24 h, with little parent lipid. Major metabolites M1, M3, M4, M6 and M7 were found across species. In vivo n=3 male rats, IV. · Efficient lipid clearance via metabolism and…; In vivo metabolism and excretion study

Methods, data and reproducibility

Male and female Sprague Dawley rats received intramuscular mRNA-1273 (Day 1 and Day 28), mRNA-1647 or NPI-Luc mRNA-LNP. mRNA, Lipid H and expressed protein were quantified in serum and tissues, followed by noncompartmental PK/PD analysis and first-order elimination simulation.

  • mRNA-1273 biodistribution study, two IM doses, tissues and blood collected to 336 h · Rat (Sprague Dawley) · n = 4 animals/sex/timepoint/dosing occasion
  • NPI-Luc reporter mRNA in Lipid H LNP, single IM dose, collection to 168 h · Rat (Sprague Dawley) · n = 3 animals/sex/timepoint
  • mRNA-1647 (6 mRNA constructs), single IM dose, male-only study, collection to 120 h · Rat (Sprague Dawley) · n = 5 animals/sex/timepoint
  • In vivo metabolism and excretion of NT-FIX mRNA in Lipid H LNP, IV infusion, plasma, urine, bile · Rat (Sprague Dawley, male) · n = n=3
  • In vitro Lipid H metabolite profiling in primary hepatocytes · Human, rat, monkey
Methods & tools
Phoenix PK (noncompartmental analysis), GraphPad Prism (simulation, t-tests), RT-qPCR with TaqMan probes, QuantiGene 2.0 bDNA assay, QuantiGene Plex multiplex bDNA (Luminex), LC-MS/MS (Sciex API 5000), Agilent 1290 UPLC with 6550 QTOF, Sandwich ELISA for Spike protein, S2P binding antibody ELISA
Reagents & environment
Phoenix PK for noncompartmental analysis; GraphPad Prism for simulation and statistics (software versions not stated).
Reproducibility gaps
Supporting data available only on reasonable request to the corresponding author; NPI-Luc protein data are not shown; no code is provided.
Limitations (5)
  • Rats are not humans, and doses were supratherapeutic (>130-fold human on a body-weight basis), so quantitative exposure is likely overestimated relative to humans. · stated by the authors
  • Last collections were up to 14 days post-dose, so late-phase elimination was estimated by first-order modeling rather than measured. · stated by the authors
  • The Spike ELISA detects only free antigen; immune complexes after the first dose likely masked Spike after the booster, so AUEClast was not estimable after dose 2 and orthogonal methods (e.g., LC-MS) are needed. · stated by the authors
  • Distal-tissue mRNA signals were near the LLOQ and highly variable across sexes, timepoints and studies, and sometimes appeared without Lipid H. · noted in this summary
  • Cross-product comparison is confounded by differing mRNA assays (RT-qPCR vs bDNA), dose levels, sampling schedules and sexes (mRNA-1647 male only). Authors are Moderna employees and shareholders, and supporting data are available only on request. · noted in this summary

Read this if you need rat biodistribution, clearance and Spike protein kinetics for mRNA-LNP vaccines to support bridging arguments across products sharing an LNP composition.

Citations
4
FWCI
8.32
Field percentile
top 2%

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