**3. Mechanisms for ligand-induced partial agonist design**

222 Steroids – Basic Science

dominated by the search for ligands characterized by partial agonistic or partial antagonistic responses, the so called selective modulators. It is hoped and expected that partial agonists and antagonists for the various receptors will provide improved therapeutic profiles. For example, selective GR modulators (SGRMs) could provide their anti-inflammatory action without the undesirable side-effects, including osteoporosis and diabetes, currently associated with oral glucocorticoids (Hudson, Roach, and Higuchi, 2008). Selective ER modulators (SERMs) hold the promise of being active on bone but not breast or endometrial tissue (Shelly et al, 2008;Silverman, 2010), whereas a desirable profile for a selective AR modulator (SARM) would likely have a greater action in bone and muscle compared to the

The shared domain structure of steroid receptors includes a variable N-terminal domain, a highly conserved DNA-binding domain and a moderately conserved ligand-binding domain (LBD). The LBD domain tends to be the primary target for drug-design. The LBD combines a number of functions, including hormone binding, receptor dimerization and binding to other co-modulating proteins that play a role in the control of transcription. These functions have the ability to influence each other, with ligand-binding, as an example; influencing the pattern of co-modulator recruitment. Specifically, gene activation requires the recruitment of co-modulating proteins to a region of the surface of the LBD formed by helices 3/4, 5 and 12. The position of helix-12, as we will discuss, can be influenced by the nature of the ligand bound to the receptor allowing drugs to influence the binding of comodulators and consequently gene activation and the resulting biological effects (Bourguet, Germain, and Gronemeyer, 2000;Egea, Klaholz, and Moras, 2000;Kumar and Thompson,

Understanding the molecular basis for partial agonism is hampered by the difficulty in solving the X-ray structures of steroid-receptors in general and specifically complexes including partial active ligands (Nettles et al, 2008). Full agonists stabilize the receptor, and specifically helix-12, in a conformation suited to binding co-activating proteins and full antagonists stabilize the receptor in a conformation suited to binding co-repressing proteins. The apparent reason for the difficulty in co-crystallizing partial agonists is that they do not fully stabilize the receptor in either conformation, adopting some degree of equilibrium between the two (Nettles et al, 2008;Raaijmakers, Versteegh, and Uitdehaag, 2009). This equilibrium allows partially active compounds to bind unique patterns of co-modulators compared to full agonists and antagonists, resulting in their potentially interesting biological effects. Unfortunately as a result it also renders them poorly suited to co-

The degree of partial activity (how far from either a full agonist or antagonist response) will go some way to determining the profile of co-modulators which will bind. Additionally, the ratio of co-activators compared to co-repressors in each cell type will influence the biological effect of a partial compound. In cells with a high co-activator concentration we would expect partial compounds to show a greater degree of agonistic activity compared to the same ligand in a cell with a high co-repressor concentration. The limitless combination of ligand partiality and co-modulator distribution appears to be a major contributor to the tissue

prostate (Gao and Dalton, 2007).

**2. Molecular basis for partial agonism** 

1999;Weatherman, Fletterick, and Scanlan, 1999).

crystallization studies.

selective responses of partial compounds.

In the absence of a complete record of X-ray structures of steroid receptors bound to agonists, antagonists and partially active compounds, we have to fill in the knowledge gaps with mutation studies and ligand-based structure-activity relationships (SAR). Even with this extra information, our understanding of the mechanisms underpinning the repositioning of helix-12 and the resulting spectrum of partial responses remains relatively naive, but there do appear to be a small number of approaches available to the drug designer who wishes to rationally influence the degree of agonism elicited by their compound series.


Incorporating these approaches into the optimization of steroid receptor ligands allows the drug-designer to modulate the degree of agonistic and antagonistic response their compounds induce. Pharmacologically it remains difficult to define *a priori* the precise agonistic or antagonistic efficacy (percentage effect or intrinsic activity) required for any desired indication, but it is now possible to generate a series of ligands with tuned efficacies to cover a broad range and then utilize molecular profiling approaches to select the most desirable.

The five basic approaches for generating partially active compounds have been deduced by numerous studies from all members of the steroid receptors and nuclear receptor family in total. For the purposes of this review we present a single receptor case study to demonstrate each of the five mechanisms, but wish to stress that to a greater and lesser degree all mechanisms should be applicable to all steroid receptors.
