伦敦论文代写 生物学论文 Examining The Covalent Modifier In Drug Discovery
This concept has been applied to the development of drugs such as cysteine-protease inhibitors (cathepsin and caspase inhibitors), tyrosine kinase inhibitors (EGFR inhibitors) and lipase inhibitors (MGL cysteine trappers inhibitors) but also to the development of pharmacological tools to localize, study and validate targets.
1.1.1 Covalent drug-target interaction: an orthogonal approach to design pharmacological tools or drugs.
The examples presented herein provide strong evidence that covalent modifiers can be safe and effective therapeutics. While in many instances the mechanism of inhibition was determined after efficacy was realized, one could adopt a covalent modifier approach from the beginning of a program.
One key success factor for this approach is the proper selection of the warhead moiety. Although there are examples of compounds containing very active functionality, such as aspirin (activated ester) and fosfomycin 1i (epoxide), a majority of the successful drugs contain functionality whose reactivity is attenuated to achieve targeted modulation. For example, the binding of rivastigmine 1d to acetylcholinesterase activates the carbamate toward cleavage by the active site serine of the catalytic triad. Another elegant example is finasteride 1u, which acts as a selective hydride acceptor from NADPH only when bound to 5R-reductase.65 In addition, the Cat K inhibitor odanacatib 1x highlights the reversible nucleophilic addition of an active site thiol to a nitrile. These examples illustrate how the location of the warhead within a structural motif can deliver both the desired therapeutic effect and safety profile.
Additionally, the prodrug approach is also valid but arguably more challenging. There are several drugs that utilize a masked warhead as the electrophilic component such as the H+/K+ ATPase inhibitors (exemplified by omeprazole 1n), where the reactive species is generated in the acidic environment of the stomach where the drug exercises its antisecretory effect. This target-localized formation of the reactive intermediate reduces systemic exposure and potential for off-target toxicities.66 The blockbuster drug clopidogrel 1o is converted to an active metabolite that is hypothesized to react preferentially with P2Y12 to prevent stoke.
Whether these successful drugs were discovered serendipitously or by design, we can use the insight provided by the available mechanistic and/or structural information to enable future de novo design of selective covalent modifiers. Paramount for success is the availability of detailed structural information on protein-ligand interaction, such as that derived from of X-ray crystallography, to facilitate the refinement of compound design and warhead placement. This approach is elegantly illustrated by the EGFR inhibitor 1t, where an appropriately placed Michael acceptor reacts readily with a nucleophilic amino acid side chain when facilitated by assistance from an internal basic amine moiety.
A systematic review of the known covalently modulated targets reveals several trends (Table 3, Charts 1 and 2). It is no surprise that the most prevalent covalently modified targets identified are enzymes (Chart 1). As a subset of the overall targets, the cysteine and serine residues are primarily modified, with few examples of other nucleophilic amino acid residues (Chart 2). Among the enzymes, proteases or hydrolases appear frequently. In addition, cofactor mediated enzymes are also represented. These data indicate that cofactor mediated enzymes or enzymes bearing an active site cysteine or serine represent attractive targets for covalent modification. The strategy to drug a target through employing covalent modifying approach could provide advantages under certain scenarios. There is typically a cost to improving the potency of lead structures that bind through noncovalent interactions. This endeavor must balance increases in molecule weight, lipophilicity, and hydrogen bonding functionality that can be detrimental to other important properties such as pharmacokinetics and ancillary pharmacology. In contrast, when a significant amount of binding energy is derived from the drug-protein covalent bond, there should be a reduction in the number noncovalent interactions needed to achieve desired potency. In the case of irreversible binders, drug concentrations in systemic circulation need only be available for a long enough period to achieve target coverage, potentially deemphasizing the need for a high, prolonged systemic drug load and therefore potentially mitigating off-target activity.67 Also, the half-life of the compound need not be long in order to achieve once a day or twice a day dosing. Certainly, reversible noncovalent inhibitors that display slow off-rates would also provide a similar benefit. While there will always be a healthy debate about pursuing molecules that bind covalently, this risk may be minimized by pursuing covalent modifiers that would be administered acutely or to patients with a life threatening disease.
Analysis of the pharmacodynamic needs of a particular therapy may lead one to consider irreversible covalent inhibition. For many diseases pharmacodynamic activity is correlated to the degree of target inhibition or occupancy. For therapies that require a high target occupancy for effective treatment, such as cancer or antibacterial therapeutics (where in the absence of high target coverage mutations may occur),68 irreversible covalent modulation could be the most effective means of treatment.
Conversely, there are therapeutic axes that would not benefit from complete covalent inhibition, wherein the complete shutdown of a primary pathway would lead to on-target toxicities. In these instances, irreversible covalent inhibition may not be appropriate. For example, in the case of warfarin, it is known that using the drug for an extended period of time (or at a high dose) can cause fatal bleeding. For this reason, warfarin is recommended for short-term use; when warfarin is used for long-term thrombosis therapy, patients are closely monitored.
That said, the industry is still searching for a safe and effective alternative to warfarin. Whether medicinal chemists pursue covalent or noncovalent modifiers, compounds should be selective for the desired target. This selectivity encompasses related pharmacological targets, as well as other endogenous nucleophilic moieties such as proteins, peptides (such as glutathione), and DNA. In any drug discovery program ancillary pharmacology studies are conducted to assess the potential liability for observing off-target toxicities in addition to in vitro safety studies. While selectivity criteria are identical for programs striving to develop either a covalent or noncovalent modifier, one might consider conducting studies to determine promiscuous binding earlier in a program utilizing a potentially reactive functional group.
It is interesting to consider how an organization might become better positioned to exploit covalent modification as a more general approach to drug discovery. For instance, one may consider building a focused screening set that would be populated with low molecular weight compounds that possess “low to moderately” reactive functionality. A lead identified from this collection could be optimized with information from crystallography and modeling studies. Medicinal chemists could further “fine-tune” reactivity, if needed, so covalent adduction is confined to the target protein. Of course opinions regarding an acceptable level of reactivity for a lead structure will always be defined differently throughout the industry. In addition, identification of functional groups beyond those mentioned in this review that selectively form covalent adducts could further enable this strategy.
伦敦论文代写 生物学论文 Examining The Covalent Modifier In Drug Discovery