NPH Insulin

Mechanism of Action

NPH insulin is an isophane suspension of human insulin and is categorized as an intermediate-acting insulin. It helps increase the cellular intake of glucose in the liver, adipose tissue, and skeletal muscles. It acts as basal insulin and stimulates the liver to promote hepatic glycogen synthesis, fatty acid metabolism for lipoprotein synthesis. In skeletal muscles, it promotes glycogen and protein synthesis. In adipose tissue, it helps in triglyceride synthesis and regulates lipolysis by inhibiting triglyceride hydrolysis. At the cellular level, it makes the cell membrane permeable for ions such as potassium, magnesium, and phosphorus and increases their cellular uptake.  

The intermediate-acting NPH insulin is obtained from the precipitation of recombinant synthesized human insulin with zinc in the presence of protamine, a basic polyarginine peptide. This process occurs at a neutral pH in an insulin and protamine ratio of 5: 1. This formulation method gives NPH the property of protraction, i.e., slow release of insulin from the precipitated insulin present at the injection site after subcutaneous injection.[1][2][3][4][5] After injecting the protein-insulin complex, the solvent from NPH insulin suspension diffuses into the subcutaneous tissue. These insulin crystals are stacked up like a heap at the injection depot and dissolve gradually, leading to the protracted action of NPH.

Later protamine-splitting enzymes and macrophages invade the subcutaneous tissue and dissolve the NPH insulin suspension heaps, thus releasing insulin. These varied heaps are also thought to be the cause of variability present in the pharmacodynamics and pharmacokinetics of NPH insulin. The exact mechanism of binding of preformed insulin-protamine aggregate and NPH insulin dissociation at tissue level is unknown.[1][3][5]

Pharmacodynamics and Pharmacokinetics

• The onset of action - 1 to 3 hours
• Time to Peak effect - 4 to 8 hours
• Time to reach peak effect in plasma – 6 to 10 hours
• Total duration - 14 to 24 hours       
• Distribution – Fast distribution in extracellular fluid
• Metabolism – Mainly liver. Some metabolism in kidney and muscle.
• Excretion – Urine metabolites

Administration

Crystalline NPH insulin administration is subcutaneous, not intramuscular or intravenous. NPH insulin is available in a two-phase solution, which means that apart from NPH, it has a solvent or a rapid-acting insulin solution. It comes in the form of a subcutaneous suspension or suspension pen-injector. Refer to the label of a specific product for more details regarding formulations and dose concentrations.  

The abdominal subcutaneous injection causes quicker absorption as compared to arms or thighs. The most significant advantage of NPH is that it can be included in premixed formulation with regular insulin. NPH in premixed formulations does not affect the potency and time-action profile of regular insulin. Exercise, massage, and local heat application increase NPH insulin absorption.[1][4][6]

NPH insulin pen requires adequate mixing to get complete resuspension before injecting it. It may be given once or twice daily.[7][8] Since the action profile of NPH insulin does not cover 24 hours, a twice-daily dose in the morning and evening or bedtime is the recommended regimen, which applies to the premixed formulations as well. Premixed formulations contain 70 to 75% of NPH insulin. The American Diabetes Association recommends moving to premixed formulations in well-controlled diabetes mellitus when HbA1c remains above target, the basal insulin is well titrated, or if the dose exceeds 0.5 units/kg/day.[9][6]

Adverse Effects

NPH insulin has a somewhat higher risk of hypoglycemia. Inadequate resuspension is thought to contribute to the high day-to-day variability in the pharmacodynamic and pharmacokinetic profile of NPH insulin, leading to hypoglycemia. Patients can adequately resuspend NPH by rotating the vial several times until it is uniformly cloudy[1][5][3][6]

It is often difficult to obtain the target glycosylated hemoglobin A1c due to its variable absorption and rapid peak.[10] Also, breakfast injection may not provide coverage till the evening dose, and likewise, the evening dose may not meet the insulin requirement until morning.

Nocturnal hypoglycemia: The patient is given NPH insulin in the evening. NPH Insulin peak occurs at midnight when the body does not require as much insulin and may cause nocturnal hypoglycemia.

Fasting hyperglycemia: Morning blood glucose can be high in patients who take NPH insulin in the evening. The duration of insulin action is not for its effect to last until the morning, potentially leading to fasting hyperglycemia, managed by giving NPH insulin at bedtime rather than with the evening meal. Fasting hyperglycemia can also occur with the dawn phenomenon, where insulin resistance and higher insulin need occurs after 5:00 AM.[4]

NPH insulin can cause weight gain.[11] Other side effects include peripheral edema, hypokalemia, and lipodystrophy, which may manifest as skin atrophy or hypertrophy.

Hypersensitivity reaction to insulin usually includes a local injection site reaction such as swelling or redness, but rarely life-threatening anaphylaxis can also occur.[12][13]

Contraindications

NPH insulin is contraindicated in hypersensitivity reactions. Patients may become hypersensitive to insulin formulation or any of the drug products. Allergic reactions to various allergens or additives such as zinc, protamine, and metal-cresol can occur. The patient may have pruritis, followed by erythema and rash all over the body.[14] NPH insulin is also contraindicated in patients who have repeated episodes of severe hypoglycemia. Hypoglycemia can be life-threatening.

Monitoring

NPH cannot classify as ideal basal insulin. It shows much variability in its absorption and action after subcutaneous injection.[1][3] This situation leads to fluctuations in blood glucose control and causes hypoglycemic episodes. For this reason, it requires meticulous monitoring, including fasting blood glucose and changes in HbA1c values.[7][11] NPH insulin requires electrolyte monitoring due to the risk of hypokalemia. In addition to consistent glucose monitoring, it requires monitoring the patient's physical activity because it may change the insulin requirement. Likewise, diet patterns require vigilance.

Frequent glucose monitoring is required in children on NPH insulin because of the greatly varying dose need and marked changes in insulin requirements during the growth period of children.[6]

Toxicity

NPH insulin toxicity can lead to severe hypoglycemia, which can cause seizures and even irreversible neurological deficits. It may be life-threatening or also cause death. Patients with renal or hepatic impairment are at higher risk of having hypoglycemia. Iatrogenic hypoglycemia can occur in medical facilities. Mild/moderate hypoglycemia is manageable by taking sugar-containing food or drinks. Diabetes mellitus patients and those on NPH insulin should have a source of sugar with them. Severe hypoglycemia and unconscious patients require hospitalization. Patients require treatment with an injection of glucagon or intravenous administration of glucose.

Enhancing Healthcare Team Outcomes

For a patient to achieve successful treatment with NPH insulin, an interprofessional team effort is necessary. Patient education is imperative to improve outcomes from NPH insulin treatment. Healthcare workers should teach patients how to resuspend NPH premixed insulin before injection. Health care professionals, particularly the diabetes nurse educator, should demonstrate the correct injection technique and educate patients about the importance of timing and accuracy of dosage. Regular monitoring is vital to avoid adverse effects. Patients need a detailed explanation of the side effects of insulin, and if available, a dedicated diabetic education session by a certified diabetic educator is necessary; this can be a nurse or pharmacist, preferably with specialty training in diabetes management. This interprofessional approach will drive improved patient compliance and outcomes with fewer adverse effects. [Level 5]

Patients may reduce insulin dose or miss it altogether, either because of the fear of hypoglycemia or the high cost of insulin. The high price is also a barrier to compliance. Some patients may find inflexible dose timing problematic due to social factors and work schedules. NPH insulin time-specific dosing schedule may require a lifestyle change, which may impact adherence to therapy. An interprofessional approach should be adopted, including a dietician, nurses, and primary care physician to improve outcomes. An endocrinology specialist should be involved in caring for patients with difficult to control diabetes mellitus or patients with labile blood glucose. An interprofessional approach, including clinicians, nurses, pharmacists, and dieticians, can improve the odds of successful glucose control with NPH insulin. [Level 5]

Recent clinical trials have shown that modern basal insulin analogs such as insulin glargine and insulin detemir have a lower risk of hypoglycemia and lower variability of fasting blood glucose compared to NPH insulin. But in clinical practice, these new analogs may not have better outcomes regarding the quality of life, morbidity, or cost.[10][2][6]