The potential therapeutic value of CDK6 inhibitors in critical COVID-19

While drug repurposing has been explored in the context of the current coronavirus disease 2019 (COVID-19) pandemic, the success of this strategy has been limited so far. A new study, released as a preprint on the medRxiv* server, describes a new method of identifying a promising hit among drugs already in use.

Inflammation in COVID-19

The most vexing aspect of severe or critical COVID-19 has been the unpredictability with which it hits. Numerous attempts to identify those at risk have met with limited success. However, it has been identified as being the result of immune overactivity, resulting in dysregulated inflammation, and causing acute respiratory distress syndrome (ARDS).

Hyper-responsiveness to respiratory infections is strongly related to genetic traits. Several genome-wide association studies (GWAS) have therefore been carried out to identify targets for therapeutic drug development or repurposing.

Many such GWAS have been compiled, and the results published as meta-analyses. This revealed several sites associated with this infection, and six that showed genetic associations with critical COVID-19 in particular. All six are related to antiviral mechanisms or inflammatory organ injury. As none of these are related to any known drug target, they are not useful for drug repurposing.

Study details

The current study focuses on identifying genes linked to traits associated with the disease, and not genes associated with the disease itself.

This was done by finding traits that are different in cases and controls and are linked to critical COVID-19. The researchers included 64 traits in all, relating to blood cell types, biochemical parameters in blood, and body mass index, from over 8,000 cases of critical COVID-19 in the UK Biobank database.

The traits considered were measured in samples collected years before the pandemic, but all related to the phenotypes that have been shown in critically ill COVID-19.

A second group included those with other respiratory infections (including pneumonia or influenza) and ARDS, with either hospitalization or death as a result. These individuals also showed the same traits at higher levels, as with the COVID-19 cases, compared to controls.

What were the findings?

The results in both groups included high reticulocyte counts, inflammatory markers such as C-reactive protein (CRP), glycated hemoglobin (HbA1c), and body mass index (BMI), along with low high-density lipoprotein (HDL) and low-density lipoprotein (LDL), and vitamin D concentrations in the COVID-19 patients and respiratory patients, relative to controls.

Traits and genetic drivers

In the second round, only four traits were observed, namely, BMI, neutrophil cell count, C-reactive protein, and alanine aminotransferase. GWAS for these traits were compared with earlier analyses.

This showed that the interleukin-6 (IL-6) gene was a driver of CRP and the cyclin-dependent kinase 6 (CDK6) gene of both BMI and neutrophil count. Since the first could not be confirmed in the current GWAS for CRP, they looked into the second.

They found significant correlations between single nucleotide polymorphisms (SNPs) rs42044 and rs445 in the CDK6 gene and BMI and neutrophil count. Further analysis showed that CRP, BMI, ALT (alanine aminotransferase), and neutrophil count predicted critical COVID-19, but not age.

Each of these traits showed unique variance in critical COVID-19. Further, a causal relationship was found between CRP and neutrophil count, and the prevalence of critical COVID-19.

Role of CRP and neutrophil count

CRP is typical of intense inflammation, which is characteristic of critical COVID-19 compared to less severe grades. Neutrophils are part of the innate immune response, and in these patients, the lungs are infiltrated with these cells, causing diffuse lung alveolar damage (DAD). This is the tissue injury that causes ARDS to develop.

Neutrophil DNA is extruded and this forms a sticky network called neutrophil extracellular traps (NETs) to trap the virus and help clear it. If excessive, NETosis contributes to clotting and inflammation. CDK4/6 is a specific regulator of NETosis, and its inhibition specifically prevents NET production without significant effect on other inflammatory pathways.

What are the conclusions?

This study shows a way to pick out drug targets that may have potential for the treatment of COVID-19, using genes encoding traits found in cases, that were absent in controls. That is, the researchers identified a cause-and-effect relationship between some traits and critical COVID-19. These were then subjected to GWAS to identify the trait-associated genetic markers.

“We hypothesize that the genetic drivers of a disease associated trait displays a good drug target to treat the disease.” Since CDK6 drives BMI and neutrophil count, it was identified as a possible drug target for critical COVID-19. There are already four approved drugs against this molecule, namely, abemaciclib, palbociclib, ribociclib, and trilaciclib, which offers the potential for repurposing them against COVID-19.

Though IL-6 could not be confirmed by earlier GWAS correlations, inhibitors like siltuximab, and IL-6 receptor inhibitors like tocilizumab and sanilumab are already approved drugs in use for certain conditions.

Immunomodulators and immunosuppressive drugs have alike been successful in treating patients with COVID-19-induced respiratory failure by inhibiting the overactive immune-inflammatory response. In early infection, conversely, they are harmful.

The virus first initiates early infection, characterized by lymphopenia, followed by the host response phase, where intense inflammation occurs.

At this stage, IL-6 holds center stage, triggering CRP release from liver cells to recruit macrophages and neutrophils, but also stimulating neutrophil synthesis and thus increased NETosis. However, IL-6 inhibitors impact many host immune responses.

In contrast, neutrophil inhibition may be a more selective way to treat this condition and the only way to treat it after viral entry into the host cell at present. That is, CDK4/6 inhibitors act after viral entry, but are more selective and can be given earlier than steroids or tocilizumab.

Moreover, neutrophil count is independently associated with critical COVID-19, indicating that even at this stage, its inhibition could produce therapeutic benefit.  

We therefore hypothesize that CDK4/6 inhibitors are superior to IL-6 inhibitors in the treatment of critically ill COVID-19. [They] can be given earlier in the infection than IL-6 inhibitors filling the therapeutic gap between vaccines and monoclonal antibodies in early infection and immunomodulators in the late stage.”

Empirical evidence comes from the observed benefit in breast cancer patients on CDK4/6 inhibitors who developed COVID-19.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.