Zika virus: a whistle-stop tour

Zika virus (false colour transmission
electron microscope image)

In a recent piece on mRNA vaccines[mRNA vaccine technology: industry is getting the message] , I mentioned Zika virus development as an indication for this emerging technology. only to realise my ignorance of this high profile pathogen (shameful, since I spent my post-doc years working in a school of tropical medicine)

First stop in filling the knowledge gap was a visit to the ever-useful World Health Organisation website which features a comprehensive “Zika timeline”. First discovered in 1947 in monkeys living in a Ugandan forest (which gave its name to the virus), and shortly after in species of Aedes mosquito, epidemiological studies conducted in the 60s and 80s indicated widespread human exposure to the Zika virus in Africa and Asia, with infection largely associated with no, or only mild, symptoms.

The more sinister nature of Zika infection emerged in 2007 and 2008, with the first confirmed large scale outbreak on the Micronesian island of Yap, and evidence that infection could be sexually transmitted. Further outbreaks occurred on various Pacific Islands during 2013 and 2014 and pointed to a link between Zika infection and birth defects and with Guillain–Barré  syndrome, a rare autoimmune disease which affects the nervous system. A year later, an epidemic characterized by a skin rash bit otherwise mild symptoms was reported in north-eastern Brazil, but was not recognised as being due to Zika virus.

By October 2015, an increasing numbers of microcephaly cases (newborns with small heads, indicative of abnormal brain development) were being reported. Further Zika outbreaks occurred in several South American and Caribbean countries over the following year, with the first cases (via sexual transmission) being identified in the continental United States in 2016.

Being generally asymptomatic, the prevalence of Zika infection is not easy to quantify, but mosquito-transmitted infection has been reported in over 80 countries (with transmission ongoing in over 60 of these). Over 1.3 million people are thought to have been infected in Brazil alone during the 2015 outbreak. In common with other vector-borne diseases, the spread of Zika owes something to human mobility, although social factors, principally the inability to afford protection against mosquitoes and high population density have been identified as key drivers.

The Zika virus has several properties that contribute to its ready transmission and to its devastating effect on foetal development. The virus is highly persistent in whole blood (up to 100 days) and in the male reproductive tract, allowing sexual transmission. Zika has a preference for certain cell types that facilitate the passage of infection through the placenta; animal studies suggest that viral preference extends to neural progenitor cells essential for normal cortical development. Whether Zika infection is a direct cause of Guillain–Barré  syndrome  has not been established.

As with malaria, Zika’s vulnerability lies in its dependence on mosquito vectors: and targeted insecticide use, management of standing water and the conscientious use of bed nets and repellents can significantly reduce transmission. Like malaria, control initiatives are vulnerable to political and economic factors, including climate change-related changes in mosquito distribution and abundance. Unlike malaria, Zika has a simple lifecycle; as with other flaviviruses (including the causative agents of yellow fever and Japanese encephalitis), infection should, in theory, be preventable through vaccination.

Vaccine development efforts are almost contemporary with the Zika outbreak itself, beginning in the second half of 2015 with the genetic analysis of Brazilian Zika isolates, with the first clinical study of a Zika vaccine being reported in late 2017. Around 45 candidate vaccines have been developed through academic, governmental and industrial efforts, with nine of these reaching the clinic, representing both established (inactivated virus) and experimental approaches (DNA and mRNA vaccines). The WHO’s initial requirement is for a vaccine that can be deployed in response to outbreaks with the primary goal of preventing congenital Zika syndrome through minimizing virus carriage in the immediate population.

Early clinical studies have established that vaccination can elicit aneutralizing antibody responses, although, while an accepted hallmark of flavivirus vaccine efficacy, the importance of neutralising antibodies, and the minimum levels needed to establish protection have still to be established in the context of Zika infection. Other important unknowns include the duration of effective vaccine-induced immunity and whether the reproductive tract can be protected from infection.

              

Perversely, given the untold misery arising from Zika outbreaks around the globe, the virus’s propensity for neural progenitor cells may offer a new means of treating the most aggressive and intractable form of brain tumour, glioblastoma.

Chinese researchers have found that an experimental live attenuated Zika vaccine functions as an oncolytic virotherapy [Going viral] in an animal model of glioblastoma, specifically infecting and destroying glioma stem cells thought to be responsible for the inevitable recurrence of the tumour. Elimination was also observed using glioma stem cells isolated from individual patients.

Photo credit: Credit: NIH/NIAID

Gene editing: off the bench and into the clinic

As the first sponsored CRISPR-Cas9 clinical study gets underway,
industry has high hopes that genome editing will eventually make gene
therapy a mainstream treatment for both inherited and non-inherited
diseases. 

Recent progress in gene therapy, with successful proof of concept being reported in a range of otherwise untreatable conditions and the grant of marketing authorisations for commercially-developed treatments makes it easy to forget that these successes were built on almost three decades of endeavour and regular disappointment.

Gene therapy restores aberrant or missing biological functions through the introduction of functional genes carried into the cell using engineered viruses or by zapping with electrical pulses. A more involved process is the alteration of specific nucleotide sequences within a faulty gene to restore or 
modify function- so-called “gene editing”. 

Around the turn of the millennium, certain bacterial genomes were observed to have multiple copies of repeated DNA sequences which read the same in both directions -“clustered regularly interspaced palindromic repeats” (CRISPR), which were subsequently found to be responsible for the exquisitely specific recognition of infecting viruses. In 2012, it was shown that CRISPR could be exploited to edit genes in mammalian cells, prompting an explosion in translational science, patent wrangles and an influx of venture funding. 

Reduced to its bare bones, CRISPR gene editing utilises a short piece of “guide” RNA, designed to bind to a specific sequence of DNA, complexed with an enzyme able to cut double-stranded DNA, with the bacterial enzyme “Cas9” being widely used. Once within the nucleus, the guide RNA latches onto its complementary DNA sequence, directing the enzyme to cut in the right place: the cell’s DNA repair mechanism completes the editing process. 

CRISPR-Cas9 (and other CRISPR based systems) have brought relative simplicity and low cost to gene editing, providing a powerful research tool with application across biological disciplines and a route to significant advances in agriculture and the treatment of both inherited and non-inherited disease.

American and European regulators have been understandably cautious with respect to CRISPR clinical studies. As with other forms of gene therapy, CRISPR-Cas9 editing is not without potential risk, such as unwanted alteration of DNA sequences other than the target gene sequence, or the triggering of immune responses to the Cas9 enzyme (obtained from one or other of two common skin bacteria) or through introduced RNA being recognised as being  “foreign”. 

Chinese investigators have been less troubled by the unknowns surrounding CRISPR clinical development:  first in man studies began in 2010, with around 80 subjects being treated in cancer and HIV gene editing studies by February this year. Several Chinese research groups have explored  gene editing in non-viable, and most recently, viable human embryos.  

CRISPR Therapeutics, in partnership with Vertex,  successfully addressed undisclosed FDA concerns raised in May this year and are now recruiting beta thalassemia patients for a Phase I/II gene editing study. Like sickle cell disease, beta thalassemia arises from an inherited defect in the gene encoding the oxygen-carrying protein, haemoglobin. 

Rather than directly editing the haemoglobin gene, the CRISPR-Cas9 complex (designated CTX001) is designed to alter a DNA sequence responsible for the shutdown of foetal haemoglobin production, in the hope that turning the switch back on will produce sufficient levels of functional haemoglobin. The editing process is performed ex vivo, with blood-forming stem cells being extracted from the patient, transfected with the CRISPR-Cas9 complex, expanded and then returned by infusion. 

CRISPR Therapeutics and its peers, Editas Medicine and Intellia Therapeutics have ambitions to develop gene editing therapies for a range of genetic conditions that result in metabolic dysfunction (Hurler syndrome, glycogen storage disease, alpha-1 antitrypsin deficiency, transthyretin amyloidosis) and visual impairment (Leber congenital amaurosis, Usher syndrome), as well as cystic fibrosis and Duchenne muscular dystrophy (DMD). Exonics Therapeutics recently published data indicating that their “SingleCut” CRISPR technology was able to  restore near normal levels of dystrophin expression in an animal model of DMD. 

CRISPR may prove to be more efficient and flexible means of engineering “off the shelf” (allogeneic) cancer-fighting T cell therapies, reducing the cost and complexity of current patient-specific CAR-T therapy  [ASCO Clinical Cancer Advances 2018. And the winner is...].

Pre-CRISPR era gene editing methodologies may also prove to have therapeutic application. Sangamo Therapeutics has used a zinc finger nuclease-based approach to achieve “in body” editing of a faulty gene in four individuals with Hunter syndrome, an enzyme disorder that prevents the breakdown of complex sugars, leading to harmful accumulation. A decrease in urinary complex sugar level was observed in two individuals, although not an increase in normal enzyme levels in the blood. Full interpretation of the significance of these results will take time but importantly, “in body” editing raised no safety issues. 

Clinical development of  gene editing-based therapies will continue to advance cautiously (at least in the West), but is likely to can to benefit from experience gained in viral and non-viral vector design now being exploited in other forms of gene therapy and growing regulatory familiarity with gene therapy risk-benefit assessment. 

Image credit: Credit: Jill George, NIH.

mRNA vaccine technology: industry is getting the message

Can mRNA technology take vaccine
design and development into a
post-Jennerian era?

Vaccination is second only to the provision of drinking water with respect to global public health benefit, and while  still some way short of universal  vaccine coverage, hundreds of millions of children and adults are protected each year  from formerly lethal  infections.

Vaccine development has never lacked ingenuity, but progress has arguably been evolutionary rather than revolutionary. The concept of injecting selected components of infectious agents rather than whole microorganisms would not greatly surprise Edward Jenner.

Twenty-first century vaccinology faces major challenges. Conventional manufacturing and deployment capabilities are taxed by the annual challenge of shifting influenza strains [A single shot] or in mounting a large-scale response to  “swine ‘flu”-like global pandemics; shorter development cycles are needed to tackle outbreaks of Zika and Ebola viruses, or whatever emerging pathogens tomorrow might bring; no effective vaccines exist for infections common in both developed and emerging economies, including HIV, Chlamydia, cytomegalovirus and tuberculosis.

Revolutions in vaccinology have proved elusive. The prospect of replacing expensive and complex vaccine manufacture with the injection of chemically-synthesised strands of DNA encoding one or more vaccine components has been pursued since the 1990s. Although simple in concept, and despite efforts to optimise DNA delivery, low potency and unresolved safety issues have confined DNA vaccination to a few animal health products. Synthetic peptide vaccines designed to mimic and present a desirable selection of antigenic sequences, struggle to elicit robust immune responses and confer effective protection.

High profile buy-ins by vaccine industry majors suggest that a true technological revolution might, finally, be on the horizon. Protein synthesis requires DNA code to be rewritten in the form  of another nucleic acid, messenger RNA (“mRNA”), which is then translated by ribosomes, the cell’s protein factories. That the transcription of DNA can be circumvented through direct injection of mRNA to produce the  corresponding protein has been known for decades, but the instability of mRNA,  and the complication that unmodified mRNA is itself highly immunogenic,  led to the exploration of mRNA vaccination  being sidelined by less technically demanding DNA and recombinant protein approaches.

Across the board advances in ease of delivery, chemical modification to improve stability and increased duration of protein production in vivo are rapidly making mRNA vaccination a viable proposition.  Clinical trials are underway in both infectious disease indications including influenza, Zika virus and HIV infection (the latter exploring therapeutic rather than prophylactic potential), and in solid and haematological cancers. The majority of cancer studies exploit the properties of specialised antigen-presenting cells (dendritic cells) which can be readily isolated from patients, loaded with tumour antigen encoding mRNA and then returned by infusion [Dendritic cell vaccines: back to the future].

Recent licensing agreements between mRNA vaccine developers and leading vaccine companies add to a growing list of industrial, governmental and non-for-profit collaborations aimed at leveraging  the benefits that mRNA technology might bring to infectious disease vaccination: higher immunogenicity; inherent safety and rapid, low-cost, scalable manufacture.

Pfizer’s $425 million headline collaboration with mRNA vaccine developer BioNTech is focused on building better flu vaccines which can be manufactured quickly and cheaply. Another mRNA vaccine pioneer, Translate Bio, entered in an $805 million headline agreement with Sanofi Pasteur in June covering five undisclosed infectious disease agents with the option to expand the collaboration to other pathogens. CureVac AG has infectious disease mRNA vaccine partnerships with both Sanofi Pasteur and Johnson & Johnson, while GSK and Novartis are collaborating on mRNA vaccine development.

Early clinical data obtained using directly injected flu and rabies mRNA vaccines can best be described as “modestly encouraging” and it will be several years before which, if any, of the various flavours of mRNA technology  can claim to be a viable route to cost-effective, large scale vaccination,  and/or  serve as a solution to  problem pathogens. The picture may become clearer with a second Phase I study of Moderna Therapeutics’s flu vaccine candidate and a Zika virus Phase I study due to complete before year end.

Image credit: Wikipedia (public domain image)

Tumour-treating fields: moving beyond glioblastoma treatment

Tumour treating fields: more than just
fun with physics

Glioblastoma (GBM), that most aggressive of brain cancers, is notoriously resistant to treatment and a variety of leading edge approaches, including immunotherapy, virotherapy and gene therapy  are under investigation as means of extending survival by a meaningful amount.

One of the few treatments to have gained regulatory approval in the last decade is a medical device, Optune™, a custom-made, shower cap-like array of electrodes worn constantly on a shaven scalp, which is claimed to disrupt and slow tumour growth through the application of low intensity, alternating electrical current- so-called “tumour treating fields (TTFs).

I’m probably not alone in being reminded of 19^th century quackery and the various galvanic treatments advocated by the beauty industry, but Optunes’s developer, NovoCure, a publicly traded US-based company, has managed to navigate the rocky path of clinical development to satisfy both American and European regulators, although approval was not without controversy.

Optune™ (then designated NovoTTF-100A) received FDA approval for the treatment of recurrent GBM in April 2011, on the back of clinical data that hinted at, rather than conclusively established, a degree of benefit comparable to chemotherapy, a comparison in itself made complex by differences in investigator choices of chemotherapy regimen. The FDA panel was accused of being overly influenced by pleas from GBM patients and their families during an open public hearing. Approval did come with the rider that NovoCure conduct a post-approval study to establish non-inferiority of TTF treatment to chemotherapy.

FDA approval for the treatment of newly-diagnosed GBM in conjunction with chemotherapy was granted in 2015 on the back of interim clinical data gathered from almost 700 GBM patients receiving radiotherapy and chemotherapy following surgery. The addition of TTF treatment increased the median time to progression, and overall survival, by almost three months over chemotherapy alone.

Neuro-oncologists remain divided on Optune™, being, with reason, critical of a pivotal clinical study that did not include sham treatment as a control, and the vagueness of the TTF mechanism of action, stated as involving “misalignment” of charged proteins critical to cell division. On the other hand, TTF in combination with chemotherapy is recommended in the authoritative National Comprehensive Cancer Center treatment guidelines. Despite the requirement of near constant wear (a minimum of 18 hours a day) and high treatment cost (around $20,000 per month with as yet limited reimbursement, Optune™ is currently in use by over 2,000 GBM patients in the US, Europe and Japan.

Recent data suggests that TTF may be of benefit in other forms of solid tumour. Two completed pilot studies of TTF in combination with standard chemotherapy in malignant mesothelioma and pancreatic cancer patients indicated improvements over historical progression-free survival and one year survival rates obtained with chemotherapy alone.

An apparent improvement in progression-free survival was observed in a small study conducted patients with recurrent ovarian cancer is more difficult to interpret due to difference in the treatment histories of individual patients and historical chemotherapy comparators.

Nonetheless, NovoCure is sponsoring three pivotal TTF studies in pancreatic cancer, non-small cell lung cancer (NSCLC) and in brain metastases arising from NSCLC, with final data from the latter study anticipated in 2020. TTF faces tougher competition in these indications from advances in precision chemotherapy and immuno-oncology, but should TTF even prove to be non-inferior to conventional therapies, it could find use in individuals intolerant of, or unresponsive to, other forms of cancer treatment.  Success may also serve to disarm TTF’s critics, as might elucidation of the putative mechanism(s) of action, along with the acid test of undertaking a  TTF study that includes a sham treatment control arm.

Image credit: Wikipedia(creative commons licence)

Holding back the years. Drug development takes on the ageing process.

Can the selective removal of old cells

 modify age-related conditions?

An oft-quoted Irish proverb advises “Do not resent growing old. Many are denied the privilege”.  Compared with the alternative, we should perhaps be more accepting of the physical (and cognitive) decline that time brings to all of us. But rising life expectancy has consequences, not least in healthcare costs: postponing or even reversing age-related decline has a societal importance far beyond personal vanity.

The impact of age is readily apparent at the cellular level, and a variety of alterations in the mechanisms which regulate normal cell division and function, including genetic stability; energy production, and cell-to-cell communication – collectively “cellular senescence” – have been identified. Ageing cells secrete a variety of pro-inflammatory proteins and are more abundant in failing hearts, joints and eyes. The relationship between senescence and organ dysfunction is misty, but animal studies in which an artificially-aged type of stem cell was transplanted into young mice resulted in a decrease in their physical capabilities and lifespan when compared to control animals.

Moreover, treatment with agents hypothesised to be capable of triggering programmed cell death (apoptosis) in senescent cells had a positive effect on physical function and lifespan in the stem cell-transplanted mice and on levels of secreted proteins associated with cellular senescence.

Drugs designed to selectively remove ageing cells- “senolytics”- are just beginning to make their way into clinical development. Unity Biotechnology’s strategy is the elimination of ageing cells in specific disease states. UBX0101 targets the interaction of the proteins MDM and p53, the latter being involved in cell-cycle arrest and apoptosis. A Phase I study is recruiting subjects with osteoarthritis, who will each receive a single intra-articular injection into the knee. A preclinical candidate, UBX1967 targets Bcl-2, another pro-apoptotic protein, and is intended for the treatment of age-related ophthalmological conditions.

Several other candidate senolytics are in preclinical evaluation. Two target the interaction between p53 and a DNA-binding protein: FOX04, a pro-apoptopic peptide (FOX04-DRI: Cleara Biotech), and an undisclosed small molecule (Antoxerene). Oisin Biotechnologies is developing a plasmid-based gene therapy to selectively ablate senescent cells.

Recently published results suggest that pharmaceutical intervention might be capable of bolstering ageing immune systems. RTB101, being developed by ResTORbio in partnership with Novartis has completed a Phase II study conducted in elderly volunteers. RTB101 is a combination of two drugs, both of which act on targets within a long-studied, multi-protein complex, “the mechanistic target of rapamycin” (TORC1), which is critical for the activation of protein translation.

The 264 study volunteers (65 years or over) received either the drug combination, one or other of the drugs or placebo tablets for six weeks and were then given a flu vaccination two weeks later. At 10-month follow-up, those receiving both drugs had the lowest rate of self-reported respiratory infection; blood tests indicated that the combination (but not single drug treatment) resulted in more complete TORC1 inhibition and that genes which control virus-fighting Type 1 interferon production were upregulated.  The authors postulated that other mechanisms might also contribute to this apparent reduction in “immunosenescence”.

Time to start planning for that 100^th birthday celebration? Probably not (unless centenarians run in the family), but there’s a reasonable expectation that one or more of the current development strategies in, or close to, clinical study, will eventually lead to useful medications, although perhaps in specific age-related disease states rather than universal elixirs of youth.

Photo credit: Linnaea Mallette https://www.publicdomainpictures.net

Gut instinct

It's what's inside that counts- microbiome research is 

starting to pin down how gut bacteria contribute

to a variety of disease states.

It's estimated that the number of Intestinal bacteria is roughly equal to the number of cells in our bodies and, intuitively, one would expect such a substantial and active biomass (around 200g in a 70kg adult) to have some influence on health and disease.

Increasingly sophisticated means of profiling the multitude of bacterial species found in the gut microbiome and the metabolites they produce are beginning to identify factors that contribute to inflammatory bowel disease, cardiovascular disease, metabolic diseases and how individuals respond to cancer immunotherapy. 

The role of the microbiome in gut conditions has, as might be expected, received particular attention. Inflammatory bowel disease (IBD) falls largely into two distinct types: ulcerative colitis (UC), which is confined to the large intestine; and Crohn's disease (CD), which may affect any part of the digestive tract. Both are chronic conditions characterised by periods of remission and flare, and are treatable but not curable. IBD is immune-mediated, although the trigger(s) remain largely undefined: genetics; the integrity of the gut surface, and environmental factors (diet, stress, antibiotic use) which may change the gut flora all play a part. 

Analysis of the gut microbiome in IBD sufferers suggests that both UC and CD are associated with different "microbial signatures", with certain bacterial species being present in abundance, and other species being absent or present only in low numbers. Bacterial degradation of mucin, a protective glycoprotein which lines and protects the gut is one possible contributory mechanism, as are changes in microbial fatty acid and amino acid biosynthesis. Association is by no means proof of causation- changes in the microbiome- “dysbiosis”- might result in chronic inflammation, but the reverse may also be the case.

Alteration of the microbiome through antibiotic treatment is associated with better clinical outcomes in IBD, but also appears to be a risk factor for the development of CD. There is no compelling evidence that probiotics- the "good bacteria" of marketing campaigns- are of benefit in IBD. While understandably unappealing, re-balancing the gut flora through "faecal microbial transplantation" (FMT) has been shown to be of benefit in small studies involving IBD patients. 

A recent study throws light on the relationship between one particular bacterial metabolite, ascorbate (vitamin C) and IBD. Microbially produced vitamin C does not contribute to our nutritional requirement but its presence in the gut may be significant. University of California researchers investigated 139 different microbial metabolites associated with CD for their effects on human T cells which mediate inflammatory processes. Fifteen metabolites were identified as having an effect on T cells, of which only ascorbate served as an inhibitor of T cell activation, possibly through altering glycolysis, a key energy-producing pathway.

That certain bacterial species associated with CD are capable of ascorbate production adds a little more circumstantial weight to the hypothesis, but further elucidation of the relationship between disease and gut ascorabate levels and the presence or absence of ascorbate-producing species is required before practical use can be made of this observation. 

The enzyme indoleamine-2,3-dioxygenase (IDO) has featured in a couple of my blog pieces as a target of great interest in cancer immunotherapy. IDO is an important regulator of tryptophan levels and increased IDO activity contributes to unwanted immunosuppression within the tumour microenvironment. The clinical performance of IDO inhibiting drugs in combination with established immuno-oncology agents has been less than stellar, leading to the large scale abandonment of further development [IDO a no-go: what’s up next in immuno-oncology combination therapy?]

Intriguingly and unexpectedly, IDO, or more accurately, the lack of it, has been found to have a beneficial effect on gut microbes in an animal model of obesity. IDO activity is increased in obesity and French researchers have shown that IDO action in the gut results in a "rewiring" of tryptophan metabolism which in turn impacts on insulin sensitivity, lipid metabolism in the liver, the integrity of the gut surface and levels of chronic inflammation. 

These effects are mediated through production of IL-22, a signalling protein involved in immune regulation and liver and gut epithelial cell survival. Knocking out IDO activity allows gut bacteria to direct tryptophan metabolism in a benign manner. The composition of the gut microbiome was found to be different between mice with active and inactive IDO. 

It's a leap to suppose that IDO inhibitors might eventually be repurposed as treatments for obesity, but better understanding of the interplay between "immunometabolic" mediators such as IDO and the microbiome could open up paths to better therapies or means of either preventing metabolic disease or identifying individuals at higher risk of obesity.

Photo credit: Darryl Leja, National Human Genome Research Institute, National Institutes of Health.

Targeting tooth decay

For now, brushing beats biopharma

in caries prevention

Back in the late 90s, I was involved in the licensing of an early-stage peptide drug aimed at preventing tooth decay by blocking the anchoring of the acid-producing (and enamel destroying) bacterium, Streptococcus mutans. As is not unusual in small biopharma, development was never progressed, but I’ve remained interested in the concept of caries prevention through pharmaceutical intervention. 

Tooth decay is an infectious disease, the principal villains being S.mutans, S.sobrinus and other acid-producing/acid-loving bad actors which outstrip other bacteria present on the tooth surface and modify the local environment to their advantage. S.mutans breaks down the sugars found in our diet and the resulting acid initiates and continues the destructon of enamel and dentine, an essential tooth mineral. Caries risk is influenced by a variety of factors, the most obvious being sugar consumption and oral hygiene habits, although the recent identification of highly virulent S.mutans strains by Swedish researchers associated with mutations in saliva proteins indicates that genetics may play a part.

As a bacterial disease, it’s not surprising that vaccination has been pursued as a possible means of caries prevention. Saliva contains secretory IgA (sIgA), a class of antibody evolved for the protection of mucosal surfaces and oral protection is theoretically possible providing effective levels of specific sIgA can be generated. Vaccination is also a low-cost and practical means of protecting large populations from an early age. 

Since the 70s, a multitude of potential vaccine targets expressed by both S.mutans and S.sobrinus have been identified and evaluated in a variety of formulations and different delivery routes (systemic, oral, intranasal), largely in animal models and a small number of volunteers studies. Despite more recently applied technical sophistication (including DNA vaccines and synthetic proteins combining several bacterial targets), development has never moved out of the transitional phase and the challenge of eliciting an effective, long-lasting antibody response remains. 

Almost as old a concept is “passive” immunisation- the use of bacteria-specific antibodies delivered in a mouth rinse or gel to sequester unwanted acid-producers. While the concept has been established in animal models and a few clinical studies, formulations based on mouse, bovine, chicken and even plant-grown antibodies have remained as curiosities. 

A recent approach combines advances in antibody generation to probe for targets actually present on living S.mutans  and S.sobrinus  and evolve corresponding antibody fragments which retain their specific binding properties. Fragments can be produced cheaply at large scale, and while successful in reducing caries in an animal model, formulation to give effective protection without frequent reapplication poses a formidable problem.

A variety of both natural and synthetic peptides with antimicrobial action have been evaluated as a means of ablating S.mutans and other acid-producers, although with mixed results. Clinical trials of a “specifically targeted antimicrobial peptide”- STAMP, designated C1652 and sponsored by a small US biotech company, C3J Therapeutics, are underway. C1652 was designed to act only on S.mutans to avoid unwanted effects on the oral microbiome. 

Interestingly, C1652 treatment appears to prevent S.mutans recolonization after treatment, although it’s too early to tell whether this useful property will be observed in human studies. A collaboration between Johnson & Johnson, a pharmaceutical and consumer healthcare giant,  and the University of Pennsylvania School of Dental Medicine is looking at the feasibility of low-cost antimicrobial peptide production in plants. 

Effective immunological or antimicrobial-based protection against decay probably remains decades away, but drug-based means of repairing the damage might be on the horizon. Researchers at the Dental Institute, Kings College London have shown that, tideglusib, a drug originally investigated as a potential Alzheimer’s disease treatment, acts upon stem cells present in dental pulp to stimulate dentine production. This process does occur naturally, but at a level too low to result in a robust repair. 

Tideglusib inhibits an enzyme, glycogen synthase kinase 3 (GSK-3), yhat normally serves as a brake on dentine production. When drug-loaded protein sponges were place in cavities formed in mouse teeth, sufficient dentine was produced to repair the tooth within six weeks. Optimised GSK-3 inhibitors could proof to be even more effective restoratives. 

In the meantime, the toothbrush, dentist and the fluoridation of drinking water remain our best defence against the ravages of S.mutans and its companions. 

Photo credit: Thegreenj, Wikipedia Creative Commons