Comparison of Oral Insulin in Type 1 Vs Type 2: Therapeutic Potential and Limitations

Abstract

Oral insulin represents a promising alternative to subcutaneous (SC) administration for diabetes management, with potential physiological, practical, and psychosocial benefits. Delivering insulin via the gastrointestinal route may better mimic endogenous portal circulation exposure, enhancing hepatic glucose regulation while eliminating injection-related discomfort and stigma. However, the oral route presents formidable challenges, including degradation by digestive enzymes, limited permeability across the intestinal epithelium, and high interpatient variability in absorption. This review evaluates both the therapeutic potential and limitations of oral insulin in Type 1 diabetes (T1D) and Type 2 diabetes (T2D). It outlines the mechanistic rationale for oral delivery, then details strategies to overcome gastrointestinal barriers—ranging from chemical permeation enhancers and enzyme inhibitors to advanced nanocarrier systems, device-based ingestion technologies, and mucosal absorption routes. Clinical trial evidence is critically assessed, highlighting metabolic benefits and adherence advantages in selected T2D populations, as well as potential adjunctive and immune-modulatory roles in T1D. Limitations such as low bioavailability, variable pharmacokinetics, cost-intensive manufacturing, and uncertainties over long-term safety are examined in depth. Patient-centered considerations, including treatment adherence, quality-of-life impact, and economic feasibility, are integrated with regulatory perspectives to provide a holistic evaluation. Although no oral insulin has yet achieved regulatory approval, ongoing innovation in delivery science, patient stratification, and device integration may enable future adoption. If technical and economic hurdles are overcome, oral insulin could become a transformative addition to diabetes therapy, particularly for patients reluctant to initiate or sustain injectable regimens.

Keywords

Oral insulin; Diabetes management; Gastrointestinal barriers; Nanocarriers; Permeation enhancers; Bioavailability; Type 1 diabetes; Type 2 diabetes; Clinical trials; Patient adherence

Introduction

Diabetes mellitus is a major and escalating global health burden, requiring lifelong management to prevent complications. While subcutaneous (SC) insulin remains a cornerstone of therapy, its use is associated with several drawbacks, including the discomfort of injections, treatment fatigue, risk of peripheral hyperinsulinemia, and psychological resistance to initiation. These barriers can impair adherence and delay optimal glycemic control.

Oral insulin administration offers a physiologically appealing alternative. By delivering insulin through the gastrointestinal tract, this approach could replicate the body’s natural first-pass delivery to the liver via the portal vein. This hepatic targeting has the potential to improve postprandial glucose regulation and reduce peripheral adverse effects linked to elevated systemic insulin levels. In addition, eliminating the need for needles may enhance patient acceptance, especially among those who are injection-averse or have needle phobia.

Despite these theoretical advantages, oral insulin has not yet reached the market. Decades of research have been hindered by formidable challenges, including enzymatic degradation in the digestive system, poor mucosal permeability, and unpredictable pharmacokinetics. Overcoming these obstacles requires advanced formulation technologies, absorption enhancers, and in some cases, innovative device-based delivery systems.

This review provides a comprehensive analysis of oral insulin development for both Type 1 diabetes (T1D) and Type 2 diabetes (T2D), covering its physiological rationale, technological and biological barriers, formulation strategies, and clinical trial outcomes. It also examines patient-centered benefits, safety concerns, regulatory and economic considerations, and future research directions aimed at translating oral insulin from experimental concept to clinical reality.

Physiological Basis for Oral Insulin:

Oral insulin, if efficiently absorbed, could replicate natural portal delivery, enhancing hepatic glucose suppression and reducing peripheral hyperinsulinemia seen with subcutaneous injections. This physiologic targeting may improve postprandial control, lower hypoglycemia risk, and limit weight gain—offering earlier, more effective fasting glucose control in Type 2 diabetes and better prandial regulation in Type 1 diabetes.

Barriers to Oral Delivery:

Oral insulin delivery is hindered by rapid enzymatic degradation in the stomach and intestine, poor permeability across the epithelial barrier, and obstruction by mucus and glycocalyx layers. Variable gastrointestinal conditions—such as pH, motility, and enzyme activity—further reduce absorption consistency. These factors collectively cause very low bioavailability and significant variability, complicating the achievement of reliable, safe, and effective long-term therapy.

Formulation and Delivery Strategies:

Multiple strategies aim to improve oral insulin delivery. Chemical enhancers such as medium-chain fatty acids, bile salts, surfactants, and mucoadhesive polymers temporarily increase epithelial permeability or protect insulin from gastric acid using enteric coatings; however, chronic use may cause irritation or excessive permeability. Nanotechnology carriers—including nanoparticles, liposomes, and metal-organic frameworks—shield insulin from degradation and promote uptake via receptor-mediated pathways, but clinical translation is hindered by manufacturing and scalability issues. Device-assisted capsules use miniature injectors to deliver insulin into the intestinal wall, bypassing mucosal barriers while maintaining oral administration; though promising, these remain in early development with regulatory challenges ahead. Mucosal routes like buccal or sublingual sprays offer needle-free rapid absorption for prandial control but face issues with salivary clearance, dose variability, and limited absorption area. While these approaches show potential, none have yet achieved widespread clinical approval.

CLINICAL EVIDENCE AND TRIALS

1. Type 2 Diabetes Mellitus (T2D)

Clinical research on oral insulin in T2D includes Novo Nordisk’s I338, which matched subcutaneous glargine efficacy but required 30–40 times higher doses, making production impractical. Oramed’s ORMD-0801 showed early promise but failed to meet phase 3 endpoints. Buccal spray Oral-lyn offered rapid absorption but had inconsistent glycemic effects, limited large-scale data, and palatability concerns. Device-based capsules like the RaniPill successfully delivered insulin into the intestinal wall in early safety trials, avoiding needles but functioning as GI-administered injectables rather than true mucosal absorption. Despite innovation, none of these approaches have reached regulatory approval due to efficacy, variability, or cost challenges.

2. Type 1 Diabetes Mellitus (T1D)

In T1D, oral insulin is explored mainly as a prandial adjunct to SC therapy, aiming to replicate hepatic first-pass action and reduce postprandial glucose spikes. Small studies using buccal or enteric formulations have shown modest benefits without increasing hypoglycemia, though absorption variability and meal timing challenges persist. Separately, low-dose oral insulin has been tested for immune tolerance induction in high-risk individuals. Large trials like DPT-1 and Trial Net yielded mixed outcomes, with some benefit in subgroups with high insulin autoantibody levels but no overall primary endpoint success. These immunomodulatory approaches target immune modulation rather than direct metabolic control.

Therapeutic Potential

1. In Type 2 Diabetes:

If cost and variability barriers are resolved, oral insulin could enable earlier initiation in injection-averse patients, enhance fasting glucose control through hepatic first-pass action, and reduce weight gain risk. It may also complement existing agents like metformin, SGLT2 inhibitors, and GLP-1 receptor agonists for synergistic, multi-targeted diabetes management.

2. In Type 1 Diabetes:

In T1D, oral insulin is best suited as an adjunct to SC therapy, potentially targeting hepatic glucose production post-meal without increasing peripheral insulin exposure. It also holds investigational potential for immune tolerance induction in high-risk, pre-symptomatic individuals, though clinical efficacy in this setting remains unproven.

LIMITATIONS AND CHALLENGES:

1. Pharmacokinetic Variability:

Oral insulin absorption is highly unpredictable due to differences in gastrointestinal pH, motility, enzyme activity, diet, and microbiota. Such variability complicates dose titration, raising risks of hypo- or hyperglycemia and limiting consistent HbA1c improvement, particularly in T1D closed-loop systems.

2. Low Bioavailability and Dose Burden:

Human bioavailability often remains under 10%, necessitating doses far higher than SC insulin. This increases pill burden and manufacturing demands, as seen with I338 requiring over 30 times the SC molar dose.

3. Safety Concerns with Enhancers:

Chemical permeation enhancers may cause chronic mucosal irritation or allow unwanted antigen passage. Long-term safety data use remains insufficient for regulatory confidence.

4. Gastrointestinal Tolerability:

Formulations may cause nausea, bloating, or diarrhea, affecting adherence, while device-based approaches introduce engineering risks.

5. Manufacturing and Cost:

High-dose production, advanced coatings, and nanocarriers elevate costs, limiting feasibility in resource-limited settings without major bioavailability or cost-efficiency breakthroughs.

Patient-Centered Perspectives:

1. Adherence and Quality of Life:

Oral insulin appeals strongly to patients, with studies showing preference even for multiple daily doses. By removing the need for injections, it may improve treatment initiation, overcome psychological insulin resistance, and enhance overall quality of life.

2. Psychosocial Impact:

Eliminating injections can reduce stigma, needle-related anxiety, and treatment burden, particularly benefiting younger patients. However, these advantages depend on maintaining effective glycemic control; inconsistent results could increase anxiety over unstable glucose, potentially diminishing the psychosocial gains. Balancing clinical efficacy with patient comfort is therefore critical to realizing the full value of oral insulin in diabetes care.

FUTURE DIRECTIONS:

1. Smarter Nanocarriers:

Glucose-responsive nanoparticles are being developed to release insulin selectively during postprandial glucose surges, aiming to enhance time-in-range and lower hypoglycemia risk.

2. Ligand-Mediated Transport:

Targeting endogenous receptors such as transferrin or neonatal Fc receptor (FcRn) offers a strategy to facilitate gut epithelial transcytosis, potentially increasing insulin bioavailability and reducing dose requirements.

3. Precision Phenotyping in Clinical Trials:

Future trials may focus on specific T2D subgroups—such as those with high hepatic glucose production and preserved beta-cell function—where first-pass hepatic targeting could provide significant therapeutic benefit.

4. Device Pill Integration:

Advances in device-based capsules, like the RaniPill, could allow insulin delivery directly into the intestinal wall, replicating SC pharmacokinetics while maintaining oral administration convenience, bypassing mucosal absorption barriers.

CONCLUSION

Oral insulin offers the promise of a more physiological delivery by directing insulin first to the liver via the portal circulation, potentially improving glucose regulation and reducing peripheral side effects compared to subcutaneous (SC) injections. In type 2 diabetes (T2D), it could enable earlier insulin initiation, enhance hepatic glucose suppression, and improve adherence among injection-averse individuals. In type 1 diabetes (T1D), its utility may lie in adjunctive prandial hepatic targeting and experimental immune-modulatory approaches in high-risk populations.

Despite decades of research, translation to clinical practice has been hindered by fundamental challenges: bioavailability often remains below 10% even with enhancers, inter and intra-patient pharmacokinetic variability complicates dosing, and high manufacturing costs particularly when doses exceed 30× SC equivalents limit scalability. Trials of notable candidates such as Novo Nordisk’s I338 and Oramed’s ORMD-0801 demonstrated safety and some glycemic benefit but failed to achieve consistent efficacy or economic viability. Safety concerns with chronic use of permeation enhancers and gastrointestinal tolerability issues further restrict adoption.

Advancement will require breakthroughs in nanocarrier design, ligand-mediated transport, device-assisted delivery, and precision patient selection. Aligning technological innovation with regulatory and cost-effectiveness criteria could ultimately make oral insulin a clinically transformative option for both T1D and T2D.

REFERENCES

1. Davies M, Pieber TR, Hartoft-Nielsen ML, Hansen OKH, Jabbour S, Rosenstock J. Effect of oral basal insulin versus subcutaneous insulin glargine in type 2 diabetes: a randomised, double-blind, phase 2 trial. Lancet Diabetes Endocrinol. 2019;7(6):471-481.
2. Rosenstock J, Bajaj HS, Janez A, Silverman R, Bloomgarden Z, Musa-Veloso K, et al. Efficacy and safety of oral basal insulin in type 2 diabetes. Diabetes Care. 2020;43(4):810-818.
3. Novo Nordisk. Company announcement: discontinuation of oral insulin I338 program. 2018.
4. Kidron M, Raz I, Tisch U, Hollander L, Oron Y, Bar-On H. Oral insulin delivery in animal models: efficacy and safety. Diabetes Obes Metab. 2004;6(1):43-50.
5. Oramed Pharmaceuticals. ORA-D-013-1 Phase 3 top-line results. 2023.
6. Ceglia L, Lau J, Pittas AG. Oral insulin and buccal insulin: a review. Diabetes Technol Ther. 2006;8(3):401-411.
7. Abramson A, Caffarel-Salvador E, Khang M, Dellal D, Silverstein D, Gao Y, et al. An ingestible self-orienting system for oral delivery of macromolecules. Science. 2019;363(6427):611-615.
8. Lamos EM, Younk LM, Davis SN. Oral insulin delivery: a review of recent developments. Drugs. 2016;76(11):1063-1074.
9. Skyler JS, Krischer JP, Wolfsdorf J, Cowie C, Palmer JP, Greenbaum C, et al. Effects of oral insulin in relatives of patients with type 1 diabetes: the Diabetes Prevention Trial–Type 1. Diabetes Care. 2005;28(5):1068-1076.
10. Maher S, Leonard TW, Jacobsen J, Brayden DJ. Safety and efficacy of absorption enhancers for improving oral peptide delivery. Expert Opin Drug Deliv. 2009;6(5):543-556.
11. Fonte P, Araújo F, Reis S, Sarmento B. Oral insulin delivery: how far are we? J Diabetes Sci Technol. 2013;7(2):520-531.
12. Brayden DJ, Alonso MJ. Oral delivery of peptides: opportunities and barriers. Ther Deliv. 2016;7(9):647-661.
13. Werle M, Bernkop-Schnürch A. Strategies to improve plasma half life time of peptide and protein drugs. Amino Acids. 2006;30(4):351-367.
14. Závada J, Rohanová D, Vávrová K. Oral insulin delivery: a challenge of formulation and manufacturing. Eur J Pharm Sci. 2022;170:106103.
15. Peyrot M, Rubin RR, Lauritzen T, Skovlund SE, Snoek FJ, Matthews DR, et al. Resistance to insulin therapy among patients and providers: results of the cross-national Diabetes Attitudes, Wishes, and Needs (DAWN) study. Diabetes Care. 2005;28(11):2673-2679.