Injection-Site Sarcomas in Rodent Carcinogenicity Studies: Human Relevance and Labeling Implications

Introduction

The two-year rodent carcinogenicity bioassay has been the regulatory standard to determine the carcinogenic potential of new drugs, particularly those intended for chronic administration. For drugs administered parenterally, the subcutaneous (SC) route is often selected. These bioassays complement short-term genotoxicity assays by evaluating tumorigenic potential under chronic exposure conditions. However, certain findings, notably injection-site tumors in rodents (eg, SC sarcomas), often reflect responses to local irritation or chronic inflammation rather than systemic carcinogenic potential. This review focuses on the mechanistic basis for rodent specificity of injection-site sarcomas and discusses implications for human risk assessment and drug labeling.

Species-Specific Responses

Extensive literature indicates that rats and mice are prone to developing injection-site soft-tissue tumors such as sarcomas from nongenotoxic xenobiotics and implants that are not considered human carcinogens.^8-10 Repeated SC injections of agents such as concentrated glucose, sodium chloride, water-soluble food colorings (eg, Brilliant blue FCF), surfactants, dextrans, and carboxymethylcellulose have produced sarcomas in rodents after prolonged exposure.^8,10 These tumors were consistently associated with a “persistent fibroblastic reaction” at the injection site, without systemic neoplasms, suggesting a local, nonspecific mechanism.

Similarly, SC implantation of solid foreign materials including polymers (polystyrene, polyethylene, silicone), inert plastics, metals (stainless steel, nickel, titanium, chromium), and glass induces sarcomas in rodents around implantation site, the so-called “Oppenheimer effect” or “solid-state carcinogenesis.”^12,15,16,19 Tumor formation in these cases is generally driven by the physical characteristics of an implant rather than by chemical toxicity.

A number of marketed injectable drugs including pegvisomant, octreotide, lanreotide, insulin glargine, liraglutide, and inotersen have been shown to produce mesenchymal tumors (including sarcoma) at injection sites in rodent carcinogenicity studies. ¹ Notably, there is no evidence of similar injection-site malignancies having been reported in humans, and malignant fibrous histiocytoma associated with SC drug administration is considered exceedingly rare.^2-7

Like rodents, cats are also particularly susceptible to injection-site-related sarcomas. Although rare, development of sarcomas at the injection site in cats following routine vaccination and, occasionally, following administration of pharmaceutical products have been reported. ¹⁹ These tumors are usually fibrosarcomas. This heightened susceptibility may also help explain the occurrence of ocular sarcomas in cats at the site of previous trauma. The proposed scientific basis for the tumors’ development in cats is a foreign-body reaction to materials like vaccine adjuvants (eg, aluminum compounds) or microchips, which leads to chronic inflammation, subsequent DNA damage, and cellular changes that can result in tumors. The cat’s unique biological response to this inflammatory process is what is thought to make the species particularly susceptible.

Role of Local Irritation and Chronic Inflammation

Injection-site sarcomas in rodents are generally attributed to high local drug concentrations or repeated injections at the same site, causing persistent irritation and chronic inflammation of connective tissue. In studies where injection-site tumors were observed, there was no evidence of systemic carcinogenic effects. For example, in the case of pegvisomant ¹ or insulin glargine (long-acting insulin analogue), ¹⁷ systemic exposure did not correlate with tumor formation, highlighting the localized nature of the observed tumors.

In a review of the induction of SC soft-tissue sarcomas in toxicity studies, Greaves ⁹ described a biphasic local tissue response with an initial phase involving cellular proliferation and tissue infiltration, and later, fibrosis and deposition of collagenous material. Injected substances that were readily absorbed without eliciting macrophage activation rarely produced neoplasia. In contrast, agents causing tissue injury, inflammation, and fibroblastic proliferation tended to be associated with sarcoma development.

Fibroblasts are key coordinators of cutaneous repair and chronic inflammation, driving extracellular matrix (ECM) deposition, paracrine signaling, wound contraction, and resolution of granulation tissue. ¹³ Review of experimental models of wound repairs highlight the differences between rodent and human skin repair. ¹⁴ Rodent skin contains a subdermal panniculus carnosus (PC) muscle that enables rapid wound contraction, a process driven by myofibroblast ECM traction and PC-mediated recoil. Human wounds, by contrast, rely more on re-epithelialization and matrix remodeling with comparatively less contraction. This difference shifts rodent fibroblasts toward earlier, faster provisional matrix turnover and increased expression of myofibroblast α-smooth muscle actin (α-SMA). ¹⁸ It is plausible that under chronic inflammatory conditions, increased expression of α-SMA and local release of mitogenic growth factors (eg, fibroblast growth factor) may lead to increased fibroblast proliferation leading to increased susceptibility of tumor formation in the rodents.

Labeling Implications of Injection-Site Tumors in Rodents

SC administration of various marketed drugs including long-acting injectables (LAIs), such as mipomersen, liraglutide, insulin glargine, lanreotide acetate, octreotide acetate, and pegvisomant, have been associated with injection-site neoplasms in rodent carcinogenicity studies.^1,17 Table 1 lists examples of approved drugs with injection-site tumor findings in rat carcinogenicity studies. It summarizes the labeling implications and human relevance of injection-site sarcomas described in the United States Package Insert (USPI) and Summary of Product Characteristics (SmPC) for the European Medicines Agency (EMA). Importantly, existing literature and product information do not indicate a risk of local malignancy at injection sites in human patients receiving these therapies. While these findings have been acknowledged in product labeling, they have not resulted in marketing restrictions or a label warning for human cancer risk.

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Table 1.

Labeling implications of injection-site sarcoma ^a findings.

Class	Drug	US labeling documents (USPI)		EMA labeling documents (SmPC)
		Mentioned in the label	Human relevance, MOA	Mentioned in the summary	Human relevance, MOA
Growth hormone antagonists	Octreotide	Yes	Not discussed, MOA—due to irritation and the high sensitivity of the rat	Yes	Not discussed, MOA—related to sustained irritant effects at the injection sites, appeared to be rat specific
	Lanreotide	Yes	May not be clinically relevant	Yes	May not be clinically relevant, MOA—likely due to the increased dose frequency in animals (daily)
	Pegvisomant	Yes	Not discussed, rodents are sensitive to repeated SC inj.	Yes	Relevance unknown, rodents are sensitive to repeated SC inj.
Long-acting insulin	Insulin glargine	Yes	Unknown relevance, MOA—due to chronic tissue irritation in rodents	No	No special hazard for humans based on carcinogenic potential
rhGLP-1 analog	Liraglutide	Yes	Not discussed, MOA—due to high dose conc. Near injection site	No	Not discussed
Lipid lowering ASO	Mipomersen b	Yes	Not discussed	N/A	N/A

Abbreviation: MOA, mode of action.

a

Fibrosarcoma, malignant histiocytomas, malignant fibrous histiocytomas.

b

Committee for Medicinal Products for Human Use (CHMP) refusal of the marketing authorization (long-term effect on liver and cardiovascular risk).

Feasibility of Conducting a Rodent Carcinogenicity Study Using Repeated SC Injections

Conducting a rodent carcinogenicity study using the SC route of administration poses practical challenges. Such studies will require multiple SC injections divided among the dorso-scapular, mid-dorsal, and/or dorso-lumbar regions. Generally, 8-16 separate injection sites are used in the dorsal region of the rat, depending on the size of the rat strain (eg, Wistar Han or Sprague Dawley). In the context of LAIs such as once every three-month (Q3M) injections, depot formation and associated inflammation may take longer time for full resolution, and therefore, repeated injections on the same site will not be recommended due to overlapping injection-site findings. Furthermore, sarcomas may expand into adjacent sites, making it difficult to evaluate the latency and tumor progression.

Local Versus Systemic Exposure Margins for Risk Assessment of Injection-Site Sarcoma

In the context of LAIs, when there is a positive carcinogenic effect due to injection-site tumors such as sarcomas, it is not feasible to calculate exposure data at injection sites. A noninvasive method to directly assess nonclinical or clinical local injection-site concentration at the SC depot is not currently available or feasible. Analytical methods, such as imaging with a radiolabel or mass spectrometry, can at best provide an indirect qualitative assessment. Consistent with ICH S1C (R2), ¹¹ a systemic exposure representing a large multiple of the human area under the exposure curve (AUC) can be an appropriate endpoint for dose selection for carcinogenicity studies for pharmaceuticals that have similar metabolic profiles in humans and rodents and low organ toxicity in rodents (ie, high doses are well tolerated in rodents). The systemic AUC is considered the most comprehensive pharmacokinetic endpoint since it relies on the plasma concentration of the compound and residence time in vivo. The use of systemic exposure margins has also been accepted for a number of LAI marketed products, such as cabotegravir and rilpivirine. Therefore, systemic exposure margins provide a scientifically and regulatory-accepted basis for safety assessment of injection-site sarcomas.

Summary

Rodents are uniquely predisposed to injection-site sarcomas following repeated SC administration of nongenotoxic agents or implantation of inert materials. These injection-site tumors are often a result of species-specific responses to chronic irritation and lack relevance to human carcinogenic risk. Considering both species-specific susceptibility and practical challenges, the SC route of administration should be avoided for rodent carcinogenicity studies whenever feasible.