**1. Introduction**

220 Steroids – Basic Science

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Activity Relationship of Various Endogenous Estrogen Metabolites for Human Estrogen Receptor and Subtypes: Insights into the Structural Determinants Favoring a Differential Subtype Binding*. Endocrinology*, Vol.147, No.3, (September The steroid hormone receptors, the Androgen Receptor (AR), Estrogen Receptors (ERα and ERβ), Glucocorticoid Receptor (GR), Mineralocorticoid Receptor and Progesterone Receptor (PR), have been crucial targets for drug discovery even before their existence was known or understood. The drugs on the market for this sub-class of the nuclear hormone receptors constitute a significant pharmacopeia for the treatment of a vast array of conditions and ailments. Despite the breadth of drugs targeted toward this family, they remain an important target for the pharmaceutical industry.

Key considerations when designing drugs for any family, beyond the on-target pharmaceutical action and safety, is to ensure specificity against related targets, exploration of the most appropriate routes of administration and desirable pharmacokinetic (PK) profiles. Developing non-steroidal modulators for the steroid receptor family has been a key strategy employed to achieve these goals, although there appears to be growing consensus that not being steroidal is insufficient to justify new drugs on its own (Hermkens et al, 2006). Unlike targeting many families, steroid hormone receptor drug discovery also has to balance the need to elicit either agonistic or antagonistic responses depending on the desired indication.

The history of drug discovery for the steroid hormone receptors has tended to follow a common path, beginning with the application of purified endogenous hormone and followed by the application of the first synthetic analogs with improved PK properties or selectivity. For some of the receptors this period was followed by the design of antagonists, including non-steroidal structures. More recently, steroid hormone drug discovery has been

Drug Design Approaches to Manipulate the Agonist-Antagonist Equilibrium in Steroid Receptors 223

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

3. Influence the position of Helix-12 by modulating the end of Helix-11 and the loop

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

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

Steroidal anti-progestins are typically differentiated structurally from progestins by the presence of a bulky attachment at their position 11 (Madauss, Stewart, and Williams, 2007). Recent publications of the anti-progestin Mifepristone (Raaijmakers, Versteegh, and Uitdehaag, 2009) and the SPRM Asoprisnil (Madauss et al, 2007) clearly demonstrate that the role of this bulky attachment is to clash with helix-12 and preclude it from adopting its required agonistic position. Both studies also demonstrate an important role specifically for Met909 in the agonism/antagonism balance. Met909 sits within helix-12 at the Cterminal end of the ligand binding domain (LBD), and in the classic agonist conformation of the receptor, is oriented toward the ligand binding pocket. Met909 is typically the only helix-12 residue directly in contact with ligands. The nature of the ligand-Met909 interactions appears to be a key determinant of the receptors function (Petit-Topin et al, 2009). Clashes between Met909 and ligands are likely to destabilize helix-12 (Raaijmakers, Versteegh, and Uitdehaag, 2009), which results in a reduced agonistic response. It has even been suggested that the degree of clash with Met909 might correspond directly to the reduction in agonism (Madauss, Stewart, and Williams, 2007), but this has yet to be

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

1. Sterically impede the agonistic orientation of Helix-12 2. Disrupt the function of other indirect stabilizing interactions.

mechanisms should be applicable to all steroid receptors.

**3.1.1 Case study: the progesterone receptor** 

**3.1 Sterically impede the agonistic orientation of helix-12** 

4. Reduce the stabilizing interactions between the ligand and Helix-12. 5. Straighten Helix-3, and/or disrupt interactions between Helices 3 & 5.

compound series.

desirable.

shown categorically.

between Helices 11 & 12.

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 prostate (Gao and Dalton, 2007).
