Zombie cells (a nickname for senescent cells) are derailed cells that wreak havoc in the body as was first demonstrated by the pioneering research by Dutch scientist Jan van Deursen. The same can be said for cancer cells. However, whereas cancer cells interfere with vital bodily functions by overgrowing healthy cells, zombie cells do not disrupt tissues and organs by multiplying. Instead, they create a toxic environment in which healthy cells cannot properly function. Despite different modus operandi, zombie cells and cancer cells have shared vulnerabilities that created a fast track for the development of drugs that eliminate zombie cells for therapeutic purposes. As the first of these drugs are bearing fruit in clinical trials for the treatment of widespread eye diseases, many others are being developed and preclinically tested. Here we look at the work that identified zombie cells as a therapeutic target for the treatment of many mainstream diseases and the development of zombie drugs, known as senolytics.
Zombie cells arise in the body’s fight against cancer
Much like we as people can get stressed from time to time, the cells in our body can get strained as well. Such cellular stress can have various sources, including oxidative stress (from radicals), genotoxic stress (damage to our genetic information), telomere shortening (the ends of our chromosomes getting dangerously short), strong mitogenic signaling (a constant push of our cells to proliferate), DNA replication errors (mistakes made when our genetic information duplicates) mitotic defects (errors made when cells actually split into two cells), and mitochondrial dysfunction (the factories that produce energy work suboptimally).
Usually, the first response of a cell to stress is to make sure that it does not divide into two daughter cells before the stress is gone. If the stress is so massive that the damage caused cannot be fixed, the cell will either die or become a zombie cell that cannot divide anymore and get usually removed later by the immune system. In both cases, the body is well served in that the damaged cells cannot go on to become cancerous later.
Zombie cells are rare but very toxic
Conversions of stressed cells into zombie cells and their subsequent removal by immune cells play out throughout life. However, this process is not perfect and as we age more and more zombie cells are not being noticed by the immune system and stick around. The absolute number of zombie cells never gets really high even as we get very old, but the problem is that these cells are very toxic to tissues and organs and will start to cause trouble. For instance, zombie cells in blood vessels can cause heart attacks and strokes, whereas in joints they can cause osteoarthritis, and in brain cognitive decline (Alzheimer’s disease). Other studies show that zombie cells in kidneys cause chronic kidney disease and that those located in the eye create macular degeneration.
All this came to light because of a new mouse model developed by Jan van Deursen and his team. This strain, dubbed INK-ATTAC allows for whole-body elimination of zombie cells. In addition to showing that the abovementioned conditions are caused by zombie cells, it revealed that zombie cells accelerate aging and death. The way that we now know how a relatively small number of zombie cells do all this is by secreting compounds that are detrimental to the cells around them. Collectively, these secreted zombie cell compounds are referred to as the senescence-associated secretory phenotype (SASP). The SASP is known to include molecules that recruit inflammatory immune cells, induce fibrosis, and inhibit the reparative functions of stem cells.
Early zombie drug discovery efforts
The exciting INK-ATTAC studies in mice inspired drug hunters to find drugs that safely kill zombie cells that drive aging and age-associated disease in humans. This endeavor had the hallmarks of a moonshot given there were no prototypes of such drugs and because the weaknesses of zombie cells were unknown at the time. One clever strategy was to take a page from the cancer drug development playbook. The reasoning to go that route was that if zombie cells are to be considered cancer-like cells that cannot form tumors because their tools to multiply have been removed, then they may have the same weak spots that cancer drug hunters used to kill cancer cells. If so, zombie cells could perhaps be eliminated with existing cancer drugs or slightly retooled cancer drugs.
While Jan van Deursen and Unity Biotechnology, the company that he cofounded, focused on the cancer playbook, James Kirkland and his team went in a different direction. In an attempt to get zombie drugs into patients and frail people on a fast track, they mainly focused on the identification of zombie drugs among vast collections of naturally occurring compounds and already approved drugs with excellent safety profiles.
The first clinically successful zombie drug
Unity Biotechnology’s strategy of repurposing well-defined cancer drugs got them onto a track to success in the clinic, although it has been a rocky road which is perhaps not all that surprising as it concerns here the development of an entirely new therapeutic. Key here are drugs that target the B cell lymphoma 2 (Bcl-2) protein family members Bcl-2, Bcl-xL, and Bcl-w, which cancer cells, but not healthy cells depend on to stay alive. Cancer drugs have therefore been developed that kill cancer cells by clamping onto the so called BH3 domain of Bcl-2, Bcl-xL, and Bcl-w.
These drugs are being used clinically for certain cancers. It turns out that zombie cells have a similar dependence on these BCL-family proteins, and these inhibitors effectively kill zombie cells in both mice and humans. For instance, in mouse models for two major age-related human diseases, atherosclerosis, and dementia, ABT-263 cleared zombie cells from atherogenic plaques and brain tissue, respectively, substantially attenuating the progression of key disease phenotype.
Using the same line of thought, small molecule drugs that interfere with the Mdm2-p53 interaction and thereby increase p53 activity can induce apoptosis in cancer cells. Unity Biotechnology explored such drugs for the killing of zombie cells in arthritic joints to alleviate pain and regrow cartilage. While the drug they developed worked beautifully in a preclinal mouse model of osteoarthritis, the use of this drug failed when tested in patients with osteoarthritis in a Phase II trial.
However, the company persisted and ultimately was successful by building on the work of Mike Sapieha, whose team in collaboration with Unity Biotechnology found that zombie cells are causing the eye disease macular degeneration, which encompasses a global patient population of about 200 million people and leads to (legal) blindness. For this disease, both the mouse model and patients with macular degeneration responded positively to the BCL2-family inhibitor UBX1325.
UBX1325 worked so well on patients for which the current standard of care (anti-VEGF treatment) failed that a single dose of UBX1325 was sufficient to provide long-lasting benefits to patients with diabetic macular edema. After the injection, in most patients the zombie cells disappeared from their eyes, the macula was spontaneously repaired, and the patients had improved vision for the duration of the trial.
Due to the success of the trial, it has now been extended to determine if patients benefit from the single shot of UBX1325 they received for up to a year. In contrast, the current standard of care requires frequent injections of anti-VGEF antibodies into the eye to prevent the disease from progressing, so a single treatment that would last for at least six months and not only halts the disease but makes the eye better would be a huge advance. A second trial on patients with wet age-related macular degeneration is well on its way with outcomes expected over the next couple of months.
Fruit and vegetable-derived zombie drugs
While Unity Biotechnology treaded carefully to avoid any possible side effects of their repurposed cancer drugs by injecting them at the side of pathology (the knee joint in the case of osteoarthritis and the eye in case of macular degeneration), James Kirkland and collaborators identified zombie drugs that had a proven safety record for oral administration and distribution throughout the body.
The two main zombie drugs that surfaced from their screenings were fisetin and quercetin. Interestingly, both are flavonoids found in fruits and veggies that have in fact been widely studied for use in cancer therapies. Quercetin has antioxidant, anti-inflammatory and immunomodulatory effects and long been used to help with a wide variety of conditions, including cardiovascular disease, arthritis, bladder infections, diabetes, and cancer but there is no compelling scientific evidence to support most of these uses. Fisetin is also known to have antioxidant and anti-inflammatory properties as well as chemopreventive and chemotherapeutic anti-cancer properties, again without clinical trials to back them up.
In contrast to BCL inhibitors, which have only one target by which to kill a cell, quercetin and fisetin have no select targets and it remains really a mystery how they exert their impact zombie cells. It could be direct by killing them through unknown mechanisms or indirectly by exerting effects on immune responses that act to eliminate zombie cells. Another way they could act would be by protecting cells against the stresses that cause them to enter into a zombie state.
Adding further complexity, quercetin works best in combination with dasatinib, which is known as a potent inhibitor of a broad spectrum of kinases. These includes SRC family kinases, receptor tyrosine kinases (EGFR, ephrin receptors, DDR1), and non-receptor tyrosine kinases (FRK, BRK, ACK) that was originally developed for Philadelphia chromosome positive chronic myeloid leukemia but has since for application in various other tumors.
A note of caution about fisetin is warranted as the initial observation (in 2018) by the Kirkland team that this drug kills senescent cells and extends lifespan could not be reproduced by three other laboratories.
While the mechanisms of action of quercetin and fisetin seem highly complex and remain subject of intense research, a myriad of studies reporting promising beneficial effects of these flavonoids in preclinical studies for the treatment of frailty, pulmonary fibrosis, muscle atrophy, osteoporosis, dementia, cardiovascular disease, and osteoarthritis, have led to the initiation of about 20 clinical trials registered on Clinicaltrails.gov. Clearly, proponents of flavonoid zombie drugs are taking a lot of shots at goal to demonstrate success of these drugs in patients and elderly citizens.
In the event of success, it will be difficult to pin the therapeutic effects on zombie cell clearance given the myriad of actions of the drugs used. However, patients with ailments for which there is currently no therapy are unlikely to care if they clearly benefit. Anyways, with the first clinical success in the books for UBX1325 use in macular degeneration, scientists, drug hunters, and investors should be inspired and keep advancing the therapeutic concept of senolysis pioneered by the Unity Biotechnology founders.