Drugging the undruggable – the story of Ras

Written by Arjun Kandola, edited by Attiya Anna Abbas and Daniele Guido.

Drug discovery and development are lengthy processes for even the most established Healthcare companies. Despite advancements in technology and ever more accessible research, commercialising new therapeutics still requires a significant amount of time and capital. Given these caveats, Healthcare companies dedicate significant resources to identify high potential drug targets within the cell. However, certain biomolecular components have proved challenging to build robust therapeutics for. Here, we explore why the Ras family of proteins are such attractive drug targets and why some Healthcare companies still have skin in the game despite four decades of failed attempts.

Ras signalling in a nutshell

Introducing cellular signalling pathways

To understand why the Ras proteins are attractive from a drug targeting perspective, we need to explore cellular signal transduction and its importance in cancer.

Signal transduction refers to the process of information being communicated through the cell, resulting in a cellular response. The signal is initiated by some internal/external stimuli and then transmitted through the cell via a series of proteins.

GTPases and cellular signalling – a match made in heaven

The Ras proteins belong to a large protein family called small GTPases. Small GTPases are useful in cellular signalling as they have an inherent ability to turn themselves ‘off’ and therefore terminate signal transduction. This is greatly beneficial as it prevents cellular responses such as growth, from remaining permanently switched on.

Figure 2) The GTP/GDP cycle. GTPase proteins bind to a molecule called Guanosine Triphosphate (GTP) and hydrolyse it into Guanosine Diphosphate (GDP). In its GTP-bound state, the GTPase’s functions are turned ‘on’, activating the next protein in the signalling pathway. After a given length of time, the GTPase hydrolyses GTP into GDP, turning the protein’s signalling function ‘off’3.

The Ras proteins and cancer

All mammalian cells express the three key Ras protein isoforms (family members): H-Ras, K-Ras, and N-Ras4. Despite the high degree of similarity between the isoforms, there are nuances in both their function (i.e., the exact signalling pathways they are involved in) and their GTP/GDP cycle dynamics. Consequently, different Ras isoforms are implicated in different diseases.

The Ras proteins are known to regulate key signalling pathways, such as those responsible for cellular survival and proliferation5. Considering how crucial these biological processes are for our body, it is not surprising that malfunctioning Ras proteins can lead to catastrophic consequences. For instance, Ras proteins locked in an active state are associated with diverse diseases including cancer4. In fact, mutations within Ras proteins are found in ca. 25% of human cancers (note, this does not imply that 25% of human cancers are driven solely by aberrant Ras protein function)4,6. Based on this statistic, it is clear that a drug that can target mutant Ras proteins could save countless lives.

Drugging the undruggable

The Ras proteins have been the subject of pharmaceutical interest for decades to no avail. In fact, many individuals within Healthcare circles considered Ras ‘undruggable’ as little as 10 years ago. Here, we outline some of the main ways researchers have attempted to target Ras.

Indirect inhibition of Ras signalling

Downstream inhibition. Targeting the downstream nodes of the Ras signal cascade would have the effect of terminating cellular response. However, this has proven difficult. For example, the RAF-MEK-ERK pathway branch (Figure 3) was found to be at the centre of a much wider signalling network with multiple inputs, outputs, and feedback mechanisms, greatly complicating drug design7.

Inhibition of Ras spatiotemporal localisation. To activate their signalling pathways, the Ras proteins must be physically bound to the inner cell membrane. This localisation is achieved by prenyl molecules attached to the Ras protein, which act as cell membrane anchors. Some research groups attempted to inhibit the attachment of cell membrane anchors to Ras proteins8. However, so called farnesyl transferase inhibitors disappointed in clinical trials7.

Direct inhibition of Ras signalling

Traditional protein inhibition. GTPases act in a similar manner to many other proteins, in that they bind a specific molecule and catalyse a reaction. In such scenarios, researchers often design ‘competitive inhibitors’ – molecules which resemble the protein’s actual substrate. When the protein binds to the competitive inhibitor its functions are inactivated. Scientists have been unable to design a competitive inhibitor for Ras due to the protein’s strong affinity for GTP over other molecules9.

Mutant specific inhibition. The presence of three Ras isoforms complicates drug targeting efforts as each Ras variant can mutate in different ways. Consequently, designing broad spectrum Ras inhibitors has proven challenging. Current research suggests that the best way to approach Ras may be via highly specific drugs catering to individual mutant variants of the Ras protein. Whilst this is a cumbersome approach, so called mutant-specific Ras inhibitors have demonstrated positive results in clinical trials3.

New kids on the block – AMG510 and MRTX849

As mentioned, mutant-specific Ras inhibitors show great promise. The K-RasG12C mutant has been identified as a driver of tumour growth and is found in approximately 13% of lung cancer cases and 3% of colorectal cancers10, rendering it an attractive drug target. From a research perspective K-RasG12C is appealing for two reasons: 1) it is comparatively easier to drug than other Ras mutants10 , 2) K-RasG12C is highly dissimilar to ‘healthy’ Ras proteins, allowing for the design of drugs that do not interfere with healthy Ras signalling and therefore avoid toxicity.

Two new drugs, both in phase I/II trials, have shown promising anti-tumour effects within mice and safe clinical activity within humans11,12. Amgen’s AMG510 and Mirati Therapeutics’ MRTX849 both work by targeting the mutated part of K-RasG12C and locking the protein in the ‘off’ state thereby shutting down aberrant Ras signalling10,13. In clinical trials, AMG510 was shown to reduce tumour size in ca. 35% of patients suffering from non-small cell lung cancer, lagging slightly behind MRTX849’s corresponding 45%. AMG510 has also delivered strong clinical results in other cancers, such as colorectal14.

Several other companies have sought to capitalise on the ‘drugability’ of K-RasG12C including Janssen/Wellspring Biosciences (ARS-3248) and Eli Lilly10, who notably quit the K-RasG12C race due to the toxicity of their potential inhibitor. In any case, it is clear that the competition to develop the most effective K-RasG12C inhibitor has just started. Further research will be important to understand safety within humans and assess whether K-RasG12C inhibitors should be used in isolation or in conjunction with other therapies10.

Concluding remarks

The battle against Ras has been almost 40-years long. Whilst initial attempts to develop broad spectrum Ras inhibitors were unsuccessful, the series of iterative steps that followed have yielded fruit. This process is a testament to the collaborative nature of the life science community, and it is important to remember Amgen and Mirati Therapeutics’ new drugs are enabled only by years of failure and volumes of high-quality research.

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References

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14. Meeting Library | CodeBreak 100: Activity of AMG 510, a novel small molecule inhibitor of KRASG12C, in patients with advanced colorectal cancer. [Internet]. [cited 2021 Apr 5]. Available from: https://meetinglibrary.asco.org/record/185490/abstract

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