Mechanism of Action

Glucocorticoids have many mechanisms of action, including transactivation, transrepression, physicochemical effects on cell membranes, synchronization of cell division and tissue remodeling, cross-reaction with other steroid hormone receptors (mineralocorticoid receptor), and others. Individually, each mechanism of action leads to a balance of efficacy vs. side effects in any specific disorder to which glucocorticoids are prescribed. For example, in DMD, the anti-inflammatory effects via transrepression pathways (NF-κB inhibition) are likely responsible for much of the efficacy seen. However, other subactivities lead to side effect profiles that detract from patient quality of life, such as effects on bone structure and function, and the stimulation of muscle atrophy pathways (induction of atrogenes also via transactivation).

Vamorolone is a new first-in-human investigational steroidal drug that effectively separates a number of subactivities seen in traditional corticosteroid drugs. As such, it is often called a ‘dissociative steroid’, in that it retains subactivities associated with efficacy (transrepression, physicochemical membrane effects, synchronization of tissue remodeling), and dissociates these efficacy-associated subactivities from other subactivities more associated with detrimental side effects. For example, vamorolone is not a substrate for the two enzymes in the body that convert corticosteroid drugs into inactive prodrugs (e.g. prednisone to prednisolone) (enzymes HSD11B1, HSD11B2). Also, vamorolone has less transcriptional activity (trans-activation) than traditional corticosteroids. Vamorolone also binds the mineralocorticoid receptor as an antagonist that may be ‘heart-healthy’, in contrast to corticosteroids that do not have this activity

The different activities of vamorolone and comparison to corticosteroids is described in more detail below: Transrepression, transactivation, physicochemical membrane effects, and synchronization of tissue remodeling, and mineralocorticoid receptor antagonism.

Transactivation

Transactivation is the best-studied of the properties of glucocorticoids and all steroidal hormones and drugs (testosterone, estrogen, vitamin D, etc.), and has been more associated with side effects rather than efficacy. It involves the lipophilic steroid diffusing through the cell membrane. In the cell cytoplasm, the steroid ligand then binds to the cytoplasmic nuclear hormone receptor (glucocorticoid receptor [GR] for prednisone and cortisol).

The receptor ligand complex then translocates to the cell nucleus, where the complex seeks out specific DNA sequences (nuclear hormone enhancer elements). For glucocorticoids, the specific enhancer element is called a GRE (glucocorticoid response element). When the ligand/receptor complex is bound to the GRE, it then typically activates gene transcription, making more of the gene mRNA and protein product. As the ligand-complex typically activates the target genes, the process is called transactivation.

Prednisolone showed a dose-dependent increase in GRE-mediated transactivation activity, whereas vamorolone did not, suggesting that vamorolone may potentially have a reduced GRE-mediated side effect profile.

Transrepression

Glucocorticoids are well known to have potent anti-inflammatory activities, which are thought to be mediated by repression of a specific inflammatory pathway, called NF-κB (NF kappa B). This is known as transrepression. NF-κB complexes are activated by multiple types of pro-inflammatory molecules and pathways (eg. TNF-mediated activation). Once activated, NF-κB receptors translocate to the cell nucleus, and then bind to specific DNA sequences (NF-κB elements) near pro-inflammatory genes and repress the action of NF-κB gene activation.

NF-κB activation is recognized as one of the earliest molecular features of Duchenne muscular dystrophy. Vamorolone is optimized to retain transrepression activities, and has similar NF-κB inhibition when benchmarked against prednisolone in muscle cells in vitro.

Physicochemical membrane sub-properties

Steroids are lipophilic molecules, where they bind lipids (fat) more easily than water and able to cross plasma membranes where they can exert direct physicochemical effects on the membranes.

Duchenne muscular dystrophy is caused by dystrophin deficiency at the plasma membrane of muscle cells, causing plasma membrane instability. Normal muscle activity can cause breaks in the myofiber plasma membrane, raising serum creatine kinase levels, and causing eventual muscle weakness.

Vamorolone was shown in animal models of DMD to protect against membrane damage, whereas prednisone was found to worsen membrane injury . This suggests that vamorolone could counter-act the membrane instability due to dystrophin deficiency in DMD.

Synchronization of tissue remodeling

Very recent research has uncovered a new subactivity of glucocorticoids – namely beneficial effects on tissue remodeling and repair through a process of cell synchronization. This new subactivity has been best studied in airway epithelial cells in asthma (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3324850/ and
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3135846/). Briefly, airway cells isolated from normal individuals can repair a wound effectively, and this is done by the participating cells communicating with each other to synchronously divide and fill the wound site. Asthmatic cells seem unable to synchronously repair a wound, showing instead a chronic inflammatory state that inhibits their repair effectiveness. Glucocorticoids appear able to rescue this defect in asthmatic cells, forcing them into mitotic synchrony, and then better able to repair a wound. Recent published data has shown that this cell-cell communication is required for this effect (http://www.ncbi.nlm.nih.gov/pubmed/24669775).

In dystrophic muscle, a similar asynchrony effect has recently been demonstrated, where glucocorticoids, as well as vamorolone, are able to better synchronize regeneration of muscle (http://www.ncbi.nlm.nih.gov/pubmed/25313409).

This newly emerging data suggests that a fourth subactivity of glucocorticoids, namely synchronization of tissue remodeling, may underlie a significant proportion of the efficacy of these drugs in chronic inflammatory states. Vamorolone appears to retain this subactivity.

Mineralocorticoid receptor antagonism

The mineralocorticoid receptor has the primary role of regulating water and salt balance in the body. Aldosterone, a steroid hormone produced by the adrenal glands, is released to signals the body to keep salt (sodium), while increasing the secretion of potassium, and retention of water in the body. This can lead to increased blood pressure. Prednisone, like aldosterone, activates the mineralocorticoid receptor, leading to retention of water and higher blood pressure; one of its side effects. Vamorolone has the opposite activity as prednisone – it is an antagonist of the mineralocorticoid receptor, inactivating it. Studies in DMD patients have shown that mineralocorticoid receptor antagonists aid in preserving heart function in DMD.

Lack of modulation by HSD11B1 and HSD11B2

There are two enzymes in the body that shuttle all corticosteroids between the inactive and active state, inclusive of the body’s normal cortisol hormone, prednisone, and deflazacort). These enzymes act upon the 11-β oxygen moiety of the steroid backbone, where conversion of this to a hydroxyl (-OH group) is the active hormone or drug, or conversion to a carbonyl (=0 group) is the inactive hormone or pro-drug. The two enzymes are called 11-β hydroxysteroid dehydrogenases, and one does the forward reaction to the active hormone/drug (HSD11B1), and the other enzyme does more of the reverse reaction (HSD11B2). These enzymes control the ratio of active vs. inactive corticosteroid systemically (throughout the circulation), and also locally at sites of cell differentiation and repair. Mouse knockout of the HSD11B1 gene has shown that this enzyme activity is required to mediate the bone side effects of corticosteroids (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6698000/ ).

Vamorolone does not contain the 11-β oxygen moiety of the steroid backbone that these enzymes modulate. Thus, vamorolone is not a substrate of either HSD11B1 or HSD11B2, and cannot be modulated by these enzymes. In this manner, vamorolone is constitutively active, and is not inactivated either systemically or locally by these modulatory enzymes.