Pulmonary vasodilation in the rat by insulin in vitro could indicate potential hazard for inhaled insulin

Review: Current status of the development of inhaled insulin

The most promising alternative route of insulin administration seems to be pulmonary delivery by inhalation. For a maximal rate of absorption insulin must be applied deep into the lung, i.e., into the alveoli. A number of inhalers designed to generate an aerosol with an appropriate particle size for pulmonary delivery are currently in clinical development. The pharmacodynamic effects of insulin formulations administered via the lung are comparable to, or are even faster than, those of subcutaneously injected regular insulin or rapid-acting insulin analogues. The relative biopotency of inhaled insulin is approximately 10%, i.e., the dose of inhaled insulin must be 10 times higher than the dose applied subcutaneously in order to induce a comparable metabolic effect. Clinical trials indicate that metabolic control with this pain free route of insulin administration is at least comparable to that of subcutaneous (sc) insulin therapy. Side effects. observed in human trials, gave rise to safety concerns that have delayed development for several years. Nevertheless, recent long-term safety studies indicate that the increased stimulation of insulin antibody formation stopped after some time and that the observed changes in lung function were minor or reversible. Consequently the first application for an approval of pulmonary insulin has been submitted to the authorities. In summary, it seems as if, after several decades of research, for the first time a feasible alternative route for insulin administration is within Reach.

Airway epithelial-derived factor relaxes pulmonary vascular smooth muscle

The factors controlling the pulmonary vascular resistance under physiological conditions are poorly understood. We have previously reported on an apparent cross talk between the airway and adjacent pulmonary arterial bed where a factor likely derived from the bronchial epithelial cells reduced the magnitude of agonist-stimulated force in the vascular smooth muscle. The main purpose of this investigation was to evaluate whether bronchial epithelial cells release a pulmonary arterial smooth muscle relaxant factor. Conditioned media from SPOC-1 or BEAS-2B, a rat- and a human-derived bronchial epithelial cell line, respectively, were utilized. This media significantly relaxed precontracted adult but not fetal pulmonary arterial muscle in an oxygen tension-dependent manner. This response was mediated via soluble guanylate cyclase, involving AKT/PI3-kinase and neuronal nitric oxide synthase. Airway epithelial cell-conditioned media increased AKT phosphorylation in pulmonary smooth muscle cells (SMC) and reduced intracellular calcium change following ATP stimulation to a significantly greater extent than observed for bronchial SMC. The present data strongly support the evidence for bronchial epithelial cells releasing a stable and soluble factor capable of inducing pulmonary arterial SMC relaxation. We speculate that under physiological conditions, the maintenance of a low pulmonary vascular resistance, postnatally, is in part modulated by the airway epithelium.

Lung function changes related to diabetes mellitus

Diabetic microangiopathy targets the lung as it does other organs. Even though respiratory dysfunction in most patients with diabetes is subclinical and rarely the presenting complaint, there are several reasons why pulmonary assessment is important: (1) Pulmonary function testing noninvasively quantifies physiological reserves in a large microvascular bed that is not clinically devastated by diabetes. (2) Subclinical loss of pulmonary reserves becomes overtly debilitating under conditions of stress, such as with aging, chronic hypoxia due to lung disease or high altitude exposure, or volume overload secondary to cardiac and renal failure. (3) Unlike myocardial or skeletal muscle function, pulmonary indices are largely independent of physical fitness. (4) Interpretation of pulmonary function indices is not complicated by secondary sequelae of diabetic end-organ failure or prior therapy. Lung function could provide useful measures of the progression of systemic microangiopathy. (5) Chronic use of inhaled insulin may affect long-term pulmonary function, while preexisting pulmonary dysfunction may alter the absorption and bioavailability of inhaled insulin. This review will discuss the changes in lung function observed in diabetes, their underlying mechanisms, and their physiological and clinical implications.

Pulmonary function changes related to acute and chronic administration of inhaled insulin

The need for frequent insulin injections to achieve optimal glycemic control remains a major barrier to initiating and maintaining insulin therapy in diabetes. The inhaled route of insulin administration offers many potential advantages. However, there are ongoing concerns regarding the pulmonary safety of inhaled insulin. Published studies reporting pulmonary safety and data submitted to the Food and Drug Administration were reviewed. All studies were open-label, included adult subjects with type 1 and 2 diabetes, and excluded patients with underlying lung disease. Inhaled insulin was compared with subcutaneous insulin and oral agents. Inhaled insulin is associated with small, consistent reductions in lung function, primarily forced expiratory volume in 1 s (FEV(1)) and diffusion capacity for carbon monoxide (DL(CO)). The small reductions in lung function occurred early (within 12 weeks) and did not progress over time. The magnitudes of the reductions were 30-50 mL for FEV(1) and 0.5-1.0 standard units for DL(CO). Collectively, the data indicate that inhaled insulin is safe in studies with duration up to 4 years. The Food and Drug Administration requires monitoring of lung function on a regular basis.

Response of the lung to pulmonary insulin dosing in the rat model and effects of changes in formulation

BACKGROUND

The hope that pulmonary insulin will provide increased patient compliance and quality of life has created great interest in patients with diabetes, the medical community, and the general public. A pulmonary insulin product is becoming a reality with clinical trials indicating comparable glycemic control with no change in pulmonary function. However, the longterm effects of pulmonary insulin dosing are not known, and as more pulmonary formulations for insulin and other proteins are rapidly being developed the need for further safety data continues to grow.

METHODS

Using gene microarrays, we compared differences in the levels of mRNAs in the lung tissue of rats that were administered a subcutaneous injection or a pulmonary instillation of insulin, as well as rats receiving an pulmonary instillation of insulin and a drug delivery agent.

RESULTS

While the insulin doses achieved comparable blood glucose depression and serum insulin concentrations, 30 mRNAs were differentially regulated in response to pulmonary dosing, including 10 mRNAs associated with an immune response and four associated with the lung's response to injury, as well as ion channels and transcription factors. When disodium 8-((N-salicyloyl-2-amino-4-chloro)phenoxy)octanoate, a drug delivery agent known to facilitate pulmonary absorption, was instilled in combination with the pulmonary insulin dose, an attenuation of this response was observed.

CONCLUSIONS

These findings suggest that undesirable effects of pulmonary dosing may be avoided by changes in formulation and that further evaluation of the effects of chronic pulmonary administration of insulin is warranted.

The diabetic lung: relevance of alveolar microangiopathy for the use of inhaled insulin

The alveolar-capillary network receives the entire cardiac output and constitutes the largest microvascular organ in the body, making it highly susceptible to systemic microangiopathy. Owing to its large reserves, symptoms and disability develop later in the lung than in smaller microvasculature such as the kidney or retina despite a comparable severity of anatomic involvement. Hence, pulmonary impairment in diabetes mellitus is under-recognized. Nonetheless, respiratory autonomic neuropathy and structural derangement of the thorax and lung parenchyma develop in many asymptomatic diabetic patients; the pathophysiology parallels that in other target organs. Even subclinical loss of alveolar microvascular reserves can be quantified noninvasively from lung diffusing capacity and its components (membrane diffusing capacity and alveolar-capillary blood volume) measured at a given cardiac output at rest or during exercise. The alveolar diffusion-perfusion relation tracks the recruitment of microvascular reserves in a manner independent of physical fitness. This article addresses the importance and pathophysiologic basis of diabetic pulmonary involvement, the assessment of diabetic alveolar microangiopathy, and the relevance of this understanding for the emerging use of inhaled insulin.