LACTOLOGY FOUNDATION

Toxicological and Lactological Monograph: ATC A10AE02 – Insulin (Beef), Long-Acting Preparations

1. ATC Classification and Pharmacological Profile

ATC Code: A10AE02 Active Substance: Insulin (beef), also known as bovine insulin Pharmacotherapeutic Group: Long-acting insulin preparations of animal origin ATC Classification: Alimentary Tract and Metabolism → Drugs Used in Diabetes → Insulins and Analogues → Insulins and Analogues for Injection, Long-Acting → Insulin (Beef) Principal Therapeutic Indications: Diabetes mellitus requiring basal insulin replacement, particularly in historical treatment settings or exceptional circumstances involving continued use of animal-derived insulin.

Insulin (beef), classified under ATC A10AE02, represents a long-acting preparation of bovine-derived insulin historically used in the management of diabetes mellitus. Before the widespread introduction of recombinant human insulin and synthetic insulin analogues, animal-derived insulin preparations constituted the principal therapeutic approach to insulin replacement.

Bovine insulin is a peptide hormone consisting of 51 amino acids arranged in two polypeptide chains connected by disulfide bonds. Its primary amino acid sequence differs from human insulin at three positions, a structural distinction that influences its immunogenicity but does not substantially alter its fundamental glucose-lowering mechanism.

The prolonged pharmacological activity of bovine insulin preparations was historically achieved through pharmaceutical modifications designed to delay subcutaneous absorption. These included protamine-containing suspensions, zinc-associated crystalline formulations, and other depot preparations.

Following subcutaneous administration, gradual dissolution of the insulin depot permits sustained absorption into the systemic circulation. The onset, peak, and duration of action vary according to the formulation, injection site, administered dose, and individual physiological characteristics.

Unlike modern long-acting insulin analogues, whose extended activity may depend on engineered molecular modifications, prolonged-action bovine insulin formulations rely primarily on physicochemical alterations in pharmaceutical composition.

Bovine insulin acts through binding to the insulin receptor, a transmembrane tyrosine kinase receptor that regulates glucose and energy metabolism. Receptor activation promotes glucose uptake into insulin-sensitive tissues, stimulates glycogen and lipid synthesis, inhibits hepatic glucose production, and suppresses lipolysis.

The pharmacological activity of bovine insulin is broadly comparable to that of human insulin, although differences in receptor affinity, immunological recognition, and absorption characteristics may influence individual therapeutic responses.

Because bovine insulin is structurally distinct from endogenous human insulin, it has a greater potential to induce anti-insulin antibody formation. This feature is particularly relevant to long-term therapeutic use and represents an important distinction between animal-derived insulin preparations and modern recombinant human insulin products.

The clinical availability of bovine insulin has declined substantially in many countries, and its use is now uncommon compared with human insulin and insulin analogues. Nevertheless, certain patients may continue treatment with animal-derived preparations because of historical treatment continuity, individual therapeutic responses, or specific clinical circumstances.

From a lactological perspective, bovine insulin shares the essential physicochemical characteristics of other insulin preparations: a relatively large molecular size, hydrophilic peptide structure, and susceptibility to gastrointestinal proteolysis. These properties are central to evaluating its potential transfer into human breast milk and the likelihood of pharmacologically meaningful systemic exposure in breastfed infants.

2. Toxicological Profile

The toxicological profile of long-acting bovine insulin is primarily determined by its insulin-mediated pharmacodynamic activity and its comparatively greater immunogenic potential.

At appropriate therapeutic doses, bovine insulin produces the expected metabolic effects of insulin replacement. However, excessive exposure may cause serious disturbances of glucose and electrolyte homeostasis.

Hypoglycaemia and Dose-Dependent Toxicity

Hypoglycaemia is the principal acute toxicological risk associated with bovine insulin therapy.

Excessive insulin activity may result from dosing errors, inadequate carbohydrate intake, increased physical activity, impaired insulin clearance, or changes in insulin sensitivity.

Clinical manifestations include sweating, tremor, tachycardia, hunger, anxiety, confusion, behavioural disturbances, seizures, and loss of consciousness.

Severe hypoglycaemia may produce neurological injury and, in exceptional circumstances, fatal outcomes.

The prolonged absorption characteristics of depot bovine insulin formulations may increase the duration of hypoglycaemic episodes following overdose. Recurrent hypoglycaemia may occur because insulin continues to be released from the subcutaneous depot after initial glucose correction.

The risk of nocturnal hypoglycaemia may be influenced by the pharmacodynamic peak of individual long-acting formulations, particularly those with variable absorption profiles.

In postpartum women, insulin requirements may decrease rapidly following delivery because pregnancy-associated insulin resistance diminishes. Failure to adjust maternal insulin dosage appropriately may therefore increase susceptibility to hypoglycaemia.

Immunogenicity and Hypersensitivity Reactions

Bovine insulin differs from human insulin at three amino acid positions, creating antigenic differences that may stimulate immune recognition.

Historically, bovine insulin preparations were associated with a greater frequency of insulin antibody formation than highly purified recombinant human insulin preparations.

Anti-insulin antibodies may influence insulin pharmacokinetics by binding circulating insulin and altering the availability of free, biologically active hormone.

In some individuals, this may contribute to variable insulin requirements, delayed insulin action, or unpredictable glycaemic fluctuations.

Hypersensitivity reactions may include local erythema, pruritus, swelling, and, rarely, systemic allergic manifestations.

Severe allergic reactions are uncommon, particularly with highly purified formulations, but remain clinically relevant.

The immunogenicity of historical animal-derived insulin preparations was also influenced by manufacturing purity and the presence of residual pancreatic proteins or other impurities. Therefore, observations from older formulations should not automatically be extrapolated to all purified bovine insulin products.

Metabolic and Electrolyte Effects

Bovine insulin stimulates intracellular potassium uptake and may produce hypokalaemia when administered in excessive doses.

Severe hypokalaemia may result in muscle weakness, cardiac conduction disturbances, and arrhythmias.

Other recognised insulin-associated adverse effects include weight gain, sodium retention, peripheral oedema, and changes in adipose tissue distribution.

Repeated subcutaneous injections may produce lipohypertrophy or, less commonly, lipoatrophy. These local tissue alterations may interfere with insulin absorption and contribute to glycaemic instability.

Organ Toxicity and Long-Term Safety

Bovine insulin is not generally associated with direct hepatotoxicity, nephrotoxicity, genotoxicity, or intrinsic carcinogenicity at therapeutic doses.

However, renal or hepatic impairment may alter insulin metabolism and increase the risk of excessive pharmacodynamic activity.

Long-term safety concerns associated with historical bovine insulin therapy primarily involved immunogenicity, variable absorption, and limitations in formulation purity rather than direct chemical toxicity.

The theoretical possibility of immune responses to bovine insulin-derived peptides is relevant to lactological assessment, but evidence demonstrating clinically significant immune-mediated effects in breastfed infants following maternal bovine insulin treatment is lacking.

The principal maternal toxicological concern during lactation remains hypoglycaemia, which may compromise maternal well-being and the ability to provide safe infant care.

3. Lactation Pharmacology and Breast-Milk Exposure

Transfer of Bovine Insulin into Human Breast Milk

Insulin is a physiological constituent of human breast milk and participates in the complex hormonal and metabolic composition of mammary secretions.

Endogenous human insulin has been detected in breast milk across different stages of lactation, and its concentration may vary according to maternal metabolic status, postpartum interval, and other physiological factors.

Exogenously administered insulin may also contribute to milk insulin concentrations.

However, direct quantitative studies specifically investigating the transfer of long-acting bovine insulin into human breast milk are extremely limited or unavailable.

Consequently, the lactological pharmacokinetics of ATC A10AE02 must be evaluated primarily through established insulin physiology, molecular characteristics, and indirect evidence from other insulin preparations.

Bovine insulin has a molecular weight of approximately 5.7 kDa and is a hydrophilic peptide with limited passive membrane permeability.

These properties are expected to restrict passive diffusion across the mammary epithelium.

Nevertheless, the physiological secretion of endogenous insulin into breast milk indicates that insulin transfer cannot be explained exclusively by passive diffusion.

Active or regulated mammary transport mechanisms may contribute to milk insulin content, although their relevance to exogenous bovine insulin has not been specifically characterised.

The three amino acid differences between bovine and human insulin do not provide a sufficient basis for predicting substantially different mammary transfer, but the absence of direct comparative studies prevents definitive conclusions.

Influence of Long-Acting Pharmaceutical Formulations

The prolonged pharmacological activity of bovine insulin preparations is achieved through delayed absorption from subcutaneous depot formulations.

Protamine-containing or zinc-associated preparations release insulin gradually into the circulation.

Once absorbed, the active circulating molecule is bovine insulin, and the pharmaceutical depot itself is not expected to transfer intact into breast milk.

The prolonged absorption profile may influence maternal circulating insulin concentrations over time, but its effect on breast-milk insulin exposure has not been directly quantified.

Accordingly, no reliable formulation-specific milk-to-plasma ratios or concentration–time profiles can be established for long-acting bovine insulin.

Oral Bioavailability in the Breastfed Infant

Insulin is a peptide hormone that is susceptible to degradation by gastrointestinal proteases.

Following oral ingestion, insulin undergoes enzymatic breakdown into smaller peptides and amino acids.

As a result, systemic absorption of intact, biologically active insulin is generally extremely low.

This characteristic is central to the lactological safety assessment of bovine insulin.

Even if small amounts of bovine insulin are transferred into breast milk, clinically meaningful systemic exposure in the breastfed infant is considered unlikely because of gastrointestinal degradation and limited intestinal permeability.

However, neonatal gastrointestinal physiology differs from that of adults.

Premature infants may exhibit reduced digestive enzyme activity and developmental differences in intestinal barrier function.

These factors introduce theoretical uncertainty regarding the gastrointestinal handling of exogenous peptide hormones, although clinically significant systemic absorption of bovine insulin from human breast milk has not been demonstrated.

Relative Infant Dose and Pharmacokinetic Uncertainty

The relative infant dose (RID) is commonly calculated by comparing the estimated infant dose received through breast milk with the maternal weight-adjusted dose.

For ATC A10AE02, a reliable RID cannot be established because published quantitative breast-milk concentration data specific to bovine insulin are insufficient.

Furthermore, conventional RID calculations may have limited clinical relevance for peptide hormones with negligible oral bioavailability.

The amount of insulin potentially ingested through breast milk does not directly correspond to the amount reaching the infant's systemic circulation.

Therefore, the assessment of infant exposure should incorporate the molecular properties of bovine insulin, gastrointestinal degradation, developmental physiology, and available clinical observations.

The absence of direct pharmacokinetic data should not be interpreted as proof that bovine insulin is completely excluded from breast milk.

Rather, available pharmacological principles support the expectation of very low systemic infant exposure.

4. Effects in the Breastfed Infant and on Lactation

Effects on the Breastfed Infant

The principal theoretical concern associated with exposure to bovine insulin through breast milk is hypoglycaemia resulting from systemic absorption of biologically active insulin.

However, the peptide structure of bovine insulin and its susceptibility to gastrointestinal degradation make clinically significant systemic glucose-lowering activity unlikely following normal oral ingestion.

There are no well-established clinical reports demonstrating hypoglycaemia in breastfed infants specifically attributable to maternal treatment with long-acting bovine insulin.

Nevertheless, the absence of documented adverse effects must be interpreted cautiously because direct clinical lactation studies involving bovine insulin are scarce.

The available evidence does not permit precise estimation of the frequency of rare adverse outcomes.

Similarly, robust prospective studies evaluating infant growth, neurological development, endocrine function, or long-term metabolic outcomes following maternal bovine insulin treatment have not been identified as part of the established evidence base.

Consequently, the safety assessment relies substantially on pharmacological plausibility and experience with insulin therapy as a broader therapeutic category.

Potential Immunological Effects

Bovine insulin differs structurally from human insulin and possesses greater antigenic potential.

This raises a theoretical question concerning whether bovine insulin peptides ingested through breast milk could influence intestinal immune recognition or contribute to immune sensitisation.

Experimental research has investigated dietary bovine insulin and other bovine proteins in relation to early immune development and autoimmune mechanisms associated with type 1 diabetes.

However, these investigations do not establish that maternal administration of bovine insulin produces clinically meaningful immunological effects in breastfed infants.

Importantly, dietary exposure to bovine insulin through cow's milk or formula cannot be considered pharmacokinetically equivalent to exposure resulting from maternal parenteral bovine insulin therapy.

The quantities, molecular forms, gastrointestinal environment, and exposure pathways may differ substantially.

Therefore, hypotheses regarding immune sensitisation should not be interpreted as evidence of established harm from breastfeeding during maternal bovine insulin treatment.

Effects on Lactogenesis and Milk Production

Insulin plays an important regulatory role in mammary gland metabolism.

Adequate insulin signalling supports glucose utilisation, lipid synthesis, and other metabolic processes involved in milk production.

Maternal diabetes may be associated with delayed lactogenesis II, particularly when glycaemic control is inadequate or additional obstetric complications are present.

However, this association reflects maternal metabolic and physiological factors rather than a direct inhibitory effect of insulin therapy.

Bovine insulin is not recognised as a direct suppressor of prolactin secretion, oxytocin-mediated milk ejection, or mammary milk synthesis.

By improving maternal glycaemic control, effective insulin replacement may indirectly support successful lactation.

Nevertheless, no robust clinical studies specifically demonstrate that bovine insulin has a distinct beneficial or adverse effect on milk production compared with recombinant human insulin.

Breastfeeding and Maternal Insulin Requirements

Maternal insulin requirements frequently decrease after delivery because of the rapid reduction in pregnancy-associated insulin resistance.

Breastfeeding may additionally influence maternal glucose metabolism through the energetic demands of milk synthesis.

The magnitude of these changes varies between individuals.

Women receiving long-acting bovine insulin may therefore require postpartum dose adjustments to prevent hypoglycaemia.

Because historical bovine insulin formulations may exhibit variable absorption characteristics, careful monitoring of maternal blood glucose is particularly important.

The principal lactation-related clinical concern is maternal metabolic stability rather than direct pharmacological toxicity in the infant.

5. Lactological Safety Assessment and Clinical Recommendations

Overall Lactological Safety Classification

Lactological compatibility: Probably compatible with breastfeeding, based on indirect pharmacological evidence.

Expected systemic infant exposure: Very low.

Risk of infant hypoglycaemia through breast milk: Theoretically very low; not directly quantified.

Effect on milk production: No established direct suppressive effect.

Need for breastfeeding interruption: Not routinely indicated solely because of bovine insulin therapy.

The lactological safety assessment of long-acting bovine insulin is based primarily on the general properties of insulin as a peptide hormone, its limited expected oral bioavailability, and extensive clinical experience with insulin replacement therapy.

The structural differences between bovine and human insulin do not provide a compelling pharmacological basis for anticipating clinically significant systemic toxicity in breastfed infants following normal milk ingestion.

However, bovine insulin cannot be assigned the same level of direct clinical evidence as modern human insulin preparations because dedicated lactation pharmacokinetic and infant outcome studies are lacking.

Therefore, the designation of probable compatibility reflects a favourable mechanistic assessment rather than comprehensive product-specific clinical validation.

Recommendations for Maternal Therapy

Women receiving bovine insulin should generally maintain clinically necessary insulin treatment during breastfeeding.

Abrupt discontinuation or inappropriate dose reduction may cause significant maternal hyperglycaemia and, in insulin-dependent diabetes, increase the risk of diabetic ketoacidosis.

Postpartum insulin requirements should be reassessed promptly because insulin sensitivity may increase substantially after delivery.

Maternal glucose monitoring should guide subsequent adjustments in basal insulin dosage and administration schedules.

Special attention should be given to the possibility of nocturnal hypoglycaemia, particularly when the preparation has a pronounced or variable pharmacodynamic peak.

Adequate carbohydrate intake, recognition of hypoglycaemic symptoms, and appropriate access to glucose supplementation are essential components of maternal safety.

In patients with a history of insulin antibody formation, unpredictable glycaemic responses, or hypersensitivity reactions, therapeutic reassessment may be appropriate.

Where clinically feasible, recombinant human insulin or modern insulin analogues may be preferred because they generally offer more extensive contemporary safety data, greater availability, and improved formulation standardisation.

However, switching a metabolically stable patient from bovine insulin should not be undertaken solely on the assumption that bovine insulin is harmful during breastfeeding.

Any transition should be individualised and supervised because differences in formulation and pharmacodynamic behaviour may alter insulin requirements.

Recommendations for the Breastfed Infant

Routine interruption of breastfeeding is not considered necessary solely because the mother receives therapeutic bovine insulin.

Similarly, expressing and discarding breast milk after maternal insulin administration is not supported by the available pharmacological evidence.

Breastfeeding does not need to be synchronised with the timing of maternal insulin injections.

Routine biochemical monitoring of infant glucose concentrations is not generally indicated solely because of maternal bovine insulin treatment.

Standard paediatric monitoring should include assessment of feeding effectiveness, hydration, weight gain, and general development.

Additional evaluation may be appropriate in premature infants, medically vulnerable neonates, or infants exhibiting unexplained lethargy, poor feeding, tremor, or other symptoms potentially associated with hypoglycaemia.

Such findings should prompt a comprehensive clinical assessment rather than automatic attribution to maternal bovine insulin exposure.

Special Clinical Considerations

Premature infants: Developmental differences in gastrointestinal digestion and intestinal permeability introduce theoretical uncertainty regarding exposure to peptide hormones. Individualised assessment may be appropriate, although clinically significant systemic absorption of bovine insulin through breast milk has not been established.

Maternal insulin antibodies: Bovine insulin may stimulate antibody formation more readily than recombinant human insulin. Antibody-associated alterations in insulin pharmacokinetics may complicate maternal glycaemic control, but direct infant effects through breastfeeding have not been demonstrated.

Historical insulin formulations: Older bovine insulin preparations may differ substantially in purity, excipients, and absorption characteristics. Product-specific assessment is appropriate when historical or exceptionally sourced formulations are used.

Maternal renal or hepatic impairment: Altered insulin metabolism may increase maternal susceptibility to hypoglycaemia and require dose adjustment.

Concurrent medications: Additional antidiabetic or other systemic medications should be assessed independently for lactational safety.

Evidence Limitations and Research Priorities

The principal limitation in the lactological evaluation of ATC A10AE02 is the scarcity of direct human studies.

Quantitative data describing bovine insulin concentrations in human breast milk, milk-to-plasma ratios, and infant systemic exposure are insufficient.

Prospective studies evaluating infant outcomes during maternal treatment with long-acting bovine insulin are also lacking.

Furthermore, much of the historical clinical experience with animal-derived insulin predates contemporary standards for lactation pharmacokinetic research and adverse-event surveillance.

These limitations prevent a high-certainty, product-specific safety classification.

Future research, where clinically feasible, could investigate analytical differentiation of bovine and endogenous human insulin in breast milk, the biological activity of milk-derived bovine insulin, and potential immunological interactions within the infant gastrointestinal tract.

However, the declining clinical use of bovine insulin may limit the feasibility of large prospective studies.

In current clinical practice, the benefit–risk assessment should therefore integrate established insulin pharmacology, maternal therapeutic necessity, the absence of a demonstrated infant toxicity signal, and the availability of alternative insulin preparations.

6. Conclusion

Long-acting bovine insulin preparations classified under ATC A10AE02 represent historically important forms of basal insulin replacement therapy.

Their toxicological profile is dominated by insulin-mediated hypoglycaemia, electrolyte disturbances, and a comparatively greater potential for immunogenicity than recombinant human insulin.

From a lactological perspective, bovine insulin is expected to present a low risk to breastfed infants because of its peptide structure, susceptibility to gastrointestinal proteolysis, and very limited anticipated systemic absorption following oral ingestion.

Although insulin is a physiological constituent of human breast milk, direct quantitative evidence describing the transfer of maternally administered bovine insulin into milk remains insufficient.

Available clinical knowledge does not establish that maternal treatment with long-acting bovine insulin causes clinically significant hypoglycaemia, growth disturbances, or other adverse effects in breastfed infants.

The theoretical immunogenicity of bovine insulin warrants scientific consideration but does not constitute evidence of clinically relevant immune-mediated harm through breastfeeding.

Bovine insulin is not recognised as a direct inhibitor of lactation, and adequate maternal insulin replacement may support breastfeeding by maintaining metabolic stability.

The principal clinical priority is therefore appropriate maternal glycaemic management, including individualised postpartum dose adjustment and prevention of hypoglycaemia.

Where alternative treatments are clinically suitable, recombinant human insulin and modern insulin analogues may be preferred because of their broader contemporary clinical experience and generally more favourable immunogenicity profiles.

Nevertheless, breastfeeding interruption is not routinely justified solely because a mother is receiving bovine insulin.

Overall, long-acting bovine insulin classified under ATC A10AE02 may be regarded as probably compatible with breastfeeding, with very low expected systemic infant exposure but limited direct lactation-specific evidence. Clinical decisions should prioritise maternal metabolic stability, individual therapeutic requirements, and appropriate paediatric surveillance.