In the second of three articles charting the history of hyperthermic treatment for bladder cancer, Johannesburg-based urologist Dr Steven Cornish looks at the development of chemotherapy and immunotherapy, the physics of bladder hyperthermia and the pros and cons of bladder thermotherapy delivery
Chemotherapy for bladder cancer first came to light in the early 1960s when the drug thiotepa, an organophosphate compound, was instilled into a bladder through a urethral catheter. Thiotepa works as an alkylating agent, binding to one of the DNA strands and preventing mitosis. Developed by Cyanamid of the United States and registered in 1959, it is still used today to treat bladder cancer. Before this, agents such as podophyllum extract, phenol and glycerin had been used intravesically with a modicum of success.
Mitomycin C was isolated from Streptomyces caespitosus in the late 1950s by Japanese scientists Dr Tojyu Hata and Dr Shigetoshi Wakagi at Tokyo’s Kitasato Institute, which was Japan’s first private medical research facility and principally concerned with developing new antibiotics. In 1953 Dr Hata discovered the antibiotic leucomycin, then three years later isolated mitomycin A and B, two drugs with antibiotic and antitumoral activity. Further analysis in a more alkaline broth detected mitomycin C, which had antibiotic and even more potent anti-tumour activity. It went on to be registered for cancer therapy and to become the most commonly used chemotherapeutic agent for bladder cancer treatment.
Gemcitabine was developed in Dr Larry Hertel’s lab at Eli Lilly in the early 1980s. It was meant to be an antiviral drug, but testing revealed it killed leukaemia cells. Further research indicated it was effective against pancreatic cancer as a standalone drug. It also became used with cisplatin to treat metastatic cancer. It was registered for use in pancreatic cancer in the UK in 1995 and in the US in 1996.
Gemcitabine works by being incorporated into DNA as a faulty base. The drug masks itself from cellular repair mechanisms by allowing normal bases to be incorporated alongside the drug. The faulty base causes an irreparable error that inhibits further DNA synthesis and hence cell death is inevitable.
Intravesical gemcitabine was initially reported as a treatment option for BCG-refractory non-muscle-invasive bladder cancer patients by a group led by the urologist Dr Guido Dalbagni in 2002. Larger studies followed after 2010, initially by Dr Giuseppe Di Lorenzo. The drug has become recognised as an effective agent with relatively low toxicity.
Docetaxel was patented in 1986 and became available clinically in 1995. It is one of a family of drugs known as taxanes. Docetaxel is a more potent semi-synthetic derivative of paclitaxel, derived from extracts of the leaves of the European yew tree (Taxus baccata).
The cytotoxic activity of docetaxel is exerted by promoting and stabilising microtubule assembly, while preventing physiological microtubule disassembly. This leads to the inhibition of mitotic cell division between metaphase and anaphase. The accumulation of microtubules also induces apoptosis though this is not its main mechanism of action.
Although it has not been so widely studied as mitomycin and gemcitabine, docetaxel has recently been used in combination with gemcitabine for chemo-hyperthermia [find out more in our interview with Professor Michael O’Donnell, the Richard D Williams Professor of Urologic Oncology at the University of Iowa,
Other drugs which have been shown to be efficacious in superficial bladder cancer include doxorubicin, epirubicin and valrubicin, which are all anthracycline antibiotics. They do not seem to have been used in hyperthermic studies. A drug known as EO9 or apaziquone initially showed superior outcomes to mitomycin and gemcitabine, but seems to have died a phase II death. Etoposide, another drug used for intravesical chemotherapy, actually has reduced efficacy when combined with heat.
The role of immunotherapy
Immunotherapy deserves a mention at this point following the pioneering work in the 1970s of Dr Donald Lamm, Director of BCG Oncology and Clinical Professor of Surgery at the University of Arizona, which saw chemotherapy for the bladder displaced by BCG (Bacillus Calmette-Guerin).
I invoked the name of Imhotep from 2600 BC in my previous article. Imhotep induced infection with incision and poultice to treat tumours. The immunotherapy brigade saw this as an immune response rather than from the fever induced. William Coley and his toxin were also thought to be efficacious from an immune response rather than a thermal effect. In 1929 a study on cadavers showed that those infected with tuberculosis had a lower incidence of cancer compared with control studies. Whoever thought that TB would be useful in healthcare instead of a scourge?
The early 1970s saw an explosion in research into BCG therapy for various cancers, starting with melanoma and rapidly encompassing other tumours. Unfortunately, just like hyperthermia nearly died an ignoble death, BCG immune therapy crashed as it just did not work except in bladder cancer. Lamm surmised that “it may simply be that bladder cancer is the ideal condition for the application of this immunotherapy”.
BCG has remained the stalwart of treating recurrent superficial bladder cancer through into current times, although cracks are starting to show in its foundations as hyperthermia therapy research gains traction. BCG is coming under pressure, not only medically but economically. Its low cost and, therefore, low profitability have resulted in recurrent shortages that threaten both bladder-cancer patients and children at risk for tuberculosis and other serious infections, and everyone involved in intravesical therapy today knows of the vagaries of BCG availability. Perhaps recombinant technologies will lead to a revival of its production as profits once again could be realised more readily for the medical industry.
What does hyperthermia do?
Turning to all the advantages of heat in the fight against cancer (which I mentioned in my previous hyperthermia article), what does heating actually do? Thermotherapy has been divided into three different ranges. Firstly, there is the range 38-40°C, which is physiological; then there is 41-43°C, which is the therapeutic hyperthermic range; and, lastly, there is greater than 43°C, which is deleterious to the body.
Physiological heat
Interleukin-1 (IL-1) is produced when an infection or other immune trigger occurs. IL-1 directly affects the brain, and a signal is sent via integratory pathways to the hypothalamus – the heat regulatory centre. An efferent response from the hypothalamus will cause appropriate effector body organs to raise the body temperature.
Heat in this range has a direct cytotoxic effect on tumour cells but with minimal growth arrest, there is an increase in vascular blood flow within the tumour and a myriad of effects occur within the immune system. These include cellular enhancement – activation of natural cells, phagocytes and dendritic cells with cross-priming of CD8 T cells and improved movement of lymphocytes. In addition, heat shock proteins undergo increased production and there is an increase in cytokines, chemokines and cell adhesion molecules.
Therapeutic heat
In this thermal range, the cytotoxic effect is more profound and there is a linear relationship with the thermal input. There is a direct effect on mitosis because RNA and DNA synthesis is impaired and DNA repair mechanisms are curtailed. The vascular alterations allow improved oxygenation to the tumour tissue and improved drug delivery. The immune effects are the same as in the physiological range.
Deleterious heat
In this range, cytotoxicity is more profound and exponential. There is apoptosis and indiscriminate cell damage. The vascular changes reduce blood flow due to damage to the endothelial cells, increased wall permeability and the presence of microthrombi. The immune response wanes as heat shock proteins are reduced and all the cellular responses are damaged resulting in immunosuppression.
Hyperthermia treatment has a direct cytotoxic effect, targeting mitosis and repair mechanisms. This cytotoxicity is reversed when the heat is withdrawn.
A linear growth arrest occurs, mainly targeting the S phase of mitosis but also slowing the M phase. S phase suppression is due to prolonged reduction in DNA synthesis and a brief reduction in RNA synthesis. Tumour cells, which are not normal, evade apoptosis mechanisms by rapidly dividing because cell arrest mechanisms are blocked. This prevents the apoptotic mechanisms from coping efficiently. The slowed mitosis allows the apoptotic pathways breathing space to perform their function optimally. Interestingly, the G phase of mitosis is protected by the accumulation of heat-shock proteins.
The raised temperature interferes with protein repair mechanisms. Proteins are damaged by the concomitant chemotherapy, and this will amplify the chemotherapeutic effect by significantly enhancing cell apoptosis.
The second aspect of tumour destruction via heat is improved vascularisation. Increasing the temperature of tissues leads to vasodilation, which allows better blood flow, so more of a chemotherapeutic drug will be carried to the target tissue. As tumour tissue is already well endowed with a maximised blood supply, so going beyond the therapeutic range will cause normal tissue to steal blood flow from the cancerous tissue as it seeks to heal itself.
The raised temperature interferes with protein repair mechanisms. Proteins are damaged by the concomitant chemotherapy, and this will amplify the chemotherapeutic effect by significantly enhancing cell apoptosis.
During hyperthermia treatment, the raised temperature interferes with protein repair mechanisms. Proteins are damaged by the concomitant chemotherapy, and this will amplify the chemotherapeutic effect by significantly enhancing cell apoptosis
Improved blood flow permits increased oxygenation of cancerous tissue. Research has shown that solid cancers have regions of mild to severe hypoxia due to abnormal vascular function, with Dr Schwarz first observing in 1909 how changes in vascular function affected radiosensitivity of tissues.
The Germans continued to dominate research into the 1920s when oxygen was demonstrated as pivotal for radiosensitivity and glycolysis described in tumour cells when confronted with increased oxygen levels. Cancer cells adapt by having an altered cellular metabolism as well as increasing resistance to chemotherapeutic drugs and radiotherapy. Improved oxygenation improves tissue healing and reduces resistance to chemotherapy.
The level of immune-system enhancement is determined by the temperature of the heated tissues and the duration of the thermal therapy. The consequence of heat therapy is that the immune system adapts to augment tumouricidal activity.
The increased temperature stimulates increased activity of dendritic cells, natural killer cells and phagocytic cells. Dendritic cells can assimilate antigens to stimulate other cells useful in antitumour activity. Natural killer cells which seek out and destroy tumour cells become super-killer cells and phagocytes function better to clear up cellular debris.
There is upregulated production of heat-shock proteins and chaperone tumour-related antigens which are released by cancer cells in the presence of heat, radiotherapy, and chemotherapy. These proteins are taken up by dendritic cells which then present these antigens to CD8 T cells and macrophages. This antigen interaction stimulates production of cytokines and interleukins – proteins which are proapoptotic and proinflammatory, culminating in increased tumouricidal activity.
The heat-shock proteins attach to tumour cell walls like labels, making them easier for the immune system to target. They can also enter tumour cells, causing disruption and cell toxicity. Intercellular adhesion molecules also have their production augmented, so there is increased trafficking of lymphocytes to the target region.
The story is far more complicated than this, but for brevity and clarity, I have tried to simplify the cellular effects of hyperthermia.
The physics of bladder hyperthermia
The physics involved in bladder chemo-hyperthermia are wonderfully complicated but there are a few easy truths to assimilate. Thermotherapy has been developed to treat muscle-invasive disease but at present this falls outside the domain of the urologist, so this article concentrates only on treatment for non-muscle-invasive bladder cancer (NMIBC).
Various institutions have vied to create mathematical models of the fluid dynamics and heat transfer inside bladders but, for multiple reasons, the process is incredibly difficult.
The bladder is an organ with a complex shape, varying from an empty tetrahedron to a full pseudosphere. Its position, shape and dimensions all vary continuously depending on gender, the degree of filling and the state of adjacent organs. Trying to understand flow – which is invariably turbulent as fluid leaves and enters the bladder either from the kidneys or via the catheter, in a constantly changing shape – can be considered challenging. The changing volume during thermic therapy also affects heat transfer within the fluid. Add to that the issue that thermal gradients can be non-uniform within the fluid bathing the bladder wall due to salinity variances and bubbles of gas which have completely different heat transfer physics, and one ends up with seriously complex mathematical models.
The average bladder wall thickness in a healthy subject is 3mm for a female and 3.3mm for a male. The average depth of the urothelial layer and the lamina is about 1mm. This is the depth we focus on when treating NIMB, which includes Tis, Ta and T1 tumours. The applied heat needs to penetrate to this depth to achieve effective chemo-thermotherapy.
The technological hurdles are much easier to surmount with NMIBC using uncomplicated thermal conduction-heating solutions. Heat applied to the wall of the bladder via warmed fluid in the bladder is taken up by the process of conduction. This simply involves the transfer of heat energy from a region of higher temperature to a region of lower temperature. This process will continue until equilibrium is reached.
Within the bladder, blood is circulating at 37° and, through convection, carries heat away from the bladder wall. Therefore, the warmed solution within the bladder must be maintained at the optimum temperature to keep the superficial layer at the required temperature.
How to keep bladder contents at 43-44°C
Broadly speaking, there are two solutions for keeping bladder contents at 43-44°C.
1. Insert a warming device into the bladder – this was the basis of the first system developed in the 1980s. However, warming fluid within the bladder makes modelling of thermal gradients complicated. Urine contains salts and has varying solute concentrations as it moves around the bladder and mixes with the chemotherapeutic agent.
Using frequencies in the microwave range, RF transmitters built into the catheter will warm the urine differentially, depending on the salt concentration at a particular point in the bladder. The RF energy also penetrates the bladder wall as radiant energy and warms the tissues directly, so heat transfer is not reliant solely on conduction. This can help maintain temperature in deeper tissues to improve the chemothermal therapeutic effect but can also result in random hot spots which can damage the bladder and surrounding tissues. Cold spots can also occur, reducing effectiveness.
2. Instill pre-warmed fluid which is constantly replaced so cooling does not occur. Since the fluid bathing the bladder walls is now at a constant temperature, modelling is easier. This method relies purely on conduction to heat the tissues. Rapid circulation counteracts the turbulent flow across the surface of the bladder and creates a uniform temperature over the entire urothelium.
The downside of this method may be that the bladder wall is not warmed as well as it would be from radiant energy supplied by a microwave source. On the other hand, uniform warming means no dangerous temperature spikes in the bladder wall, while the input temperature of the fluid can be relied on as the maximum thermal temperature of the bladder tissue.
A thermal difference of only 6-7°C is considered by some researchers to be suboptimal for heating the bladder wall. However, as the temperature change occurs over a short distance, the gradient is steep and permits heat penetration of 2-3mm into the bladder wall, which should be ample for NMIBC. The safety of this system has led to rapid acceptance amongst the urological community.
Heat transfer to the bladder wall is not instantaneously in the optimum range when a system is switched on. Modelling has shown a period of changing thermal milieu until a steady state is reached, which can take up to 15 minutes.
Also important is the requirement to prevent an accumulation of heat. This is important in systems that heat the bladder contents in situ. The bladder contents need to be recycled and passed through a cooling device, otherwise runaway temperatures inside the bladder could occur. With systems that heat the fluid containing the drug outside the body, it’s easier to hold the temperature at the required level.
I have merely scratched the surface of the complex physics and engineering involved in this treatment modality, but I hope I have given the reader some understanding. Turning to the actual drugs used as the partner in chemo thermotherapy, I’ll use mitomycin C as the drug example, as it has been studied extensively.
Mitomycin C with hyperthermia
The key takeaway is that with hyperthermia there is greater drug uptake into the tissues, so you get more bang for your buck.
In the pre-hyperthermia era, mitomycin C (MMC) proved inferior to BCG. MMC plasma levels were highest shortly after TURBT, when there was a denuded surface in the bladder, yet even these levels were 10 times lower than that required for myelosuppression. As the bladder healed, plasma levels of MMC declined after each weekly instillation. Two weeks after the TURBT the plasma levels were already two to fourfold lower.
The depth of penetration followed a logarithmic curve. Roughly for each 500 microns, MMC concentration fell by 50%. The urothelium, however, constitutes the major barrier and the tissue concentrations are 30 times lower just under the mucosa than in the bladder fluid.
There are various chemical and physical methods to increase the permeability of the urothelium. In this article I will address only the physical thermal effect.
The impermeability of the bladder mucosa is due to the tight cell junctions of the umbrella cells and enhanced by the negatively charged glycosaminoglycan (GAG) layer. The GAG layer prevents diffusion of substances, while the umbrella cells will only allow active transport of certain molecules.
Heat supplies greater energy to the chemotherapeutic molecules assisting the drug to penetrate the tissues more readily. MMC crosses cell membranes by active transport. When heat is added, changes occur in the mean plasma concentration of the drug. In comparator trials the plasma concentrations reached were at least double after 30 minutes in the thermal group and this was maintained until 60 minutes. Prior to 30 minutes there was a time-dependent improvement, presumably because the applied heat caused a gradual increase in permeability of the urothelium.
It has been noted that when a tumour is present at the time of chemo-thermotherapy, the plasma levels rise even higher, presumably because of the increased vascularity of tumours. Fortunately, multiple tumours do not seem to result in a toxic dose. Furthermore, within the tumour tissue itself there is an up to 10-fold increase in drug concentration due to parameters of tumour tissue described earlier.
MMC is metabolised in the target cells to three forms that are directly cytotoxic. The drug metabolism increases proportionally by 50% for each 1°C rise in temperature, thereby enhancing cancer cell destruction. You may ask: does the heat not damage the drug? The answer seems to be no. There was almost no difference in the concentration levels of drug in the urine after treatment comparing thermal to non-thermal therapy controls.
The first local chemothermic therapy devices
Hyperthermia can be delivered as a local, regional or whole-body therapy. For bladder cancer, I will be considering local therapy only in its intravesical format. The bladder readily lends itself to this form of treatment as it is accessible from the outside and valves at the ureteric junctions and bladder neck mean the treatment can be confined solely to the bladder.
There are machines that can deliver regional bladder hyperthermia. This hardware heats the bladder from the outside and can heat the entire bladder wall and the perivesical tissue. Work is being conducted to look at treating muscle invasive bladder tumours with this treatment modality. However, it is extremely expensive and is unlikely to be purchased by a urologist.
I will now introduce the technologies developed for local chemo-hyperthermia of non-muscle-invasive bladder cancer (NMIBC).
The chemo-hyperthermia story starts in 1972, when thiotepa was used in conjunction with thermotherapy up to 44°C for low-stage bladder tumours. The initial trials were not very successful, with many side effects. In the 1980s an existing microwave technology was combined with mitomycin C to develop the first intravesical chemo-thermotherapy.
Synergo
The Synergo system was developed to improve chemo-hyperthermia in the bladder by placing the heating source inside the bladder so energy didn’t have to travel from the outside to reach the bladder. This concept was akin to brachytherapy of the prostate (instead of external beam radiation), a technique well known to urologists.
A small diameter intraluminal microwave antenna was developed that was embedded in a multi-lumen catheter. The source of the microwave radiation was thus significantly simplified, bringing the cost of the treatment down. The microwave antenna heated not only the fluid in the bladder lumen but also the bladder wall through radiation. By circulating the fluid and cooling it, the designers could achieve higher energy levels in the antenna, which improved heat penetration through the bladder fluid and into the bladder wall. The system has had several iterative improvements with time. However, due to issues already mentioned, it remained a tool mainly of thermotherapists. The majority of the early literature focused on this delivery device.
The safety aspects of the system as well its cost prompted a new approach, and the idea of introducing pre-heated chemotherapy to the bladder was considered.
Combat Medical
The Combat system circulates heated drug through the bladder. It heats a fluid containing the chemotherapeutic compound in a range of 41°C to 44°C (+/-0.2°C), using a temperature-controlled, water-bath heat source that is absolutely critical to the whole concept, plus a peristaltic pump. The fluid is pumped at low volumes through a very high efficiency heat exchanger, with temperatures consistently monitored by an inline probe. The closed circuit delivers the fluid to the bladder through a soft, three-way silicone catheter. Alarm systems monitor for over-pressure as well as temperatures outside the therapeutic range.
The major challenge faced in developing Combat was the research required to develop the heating plate for warming the mitomycin C. It took several years to overcome multiple technical hurdles. Once this aspect of the system was functioning reliably, the rest of the components proved easier to develop and integrate. The constant thermal gradient with no hot or cold spots optimises therapy for NMIBC.
Unithermia
The Unithermia system uses a similar design but a different method of introducing the thermal gradient. The manufacturer calls it bladder wall thermotherapy (BWT). In this application, the drug-containing solution is heated to 46.5°C from an outside source and circulated rapidly through the bladder, reaching four fluid changes per minute. The temperature of the fluid coming into contact with the bladder is a very uniform 44 to 44.5 ° C. The high flow allows fresh drug to be continuously presented to the bladder wall. As with Combat, the thermal gradients are safe and predictable.
Pros and cons of a bladder thermotherapy delivery system
Any treatment given to a patient is always an amalgam of benefits and side effects, and intravesical bladder therapy is no exception.
Advantages include the fact that by inserting a drug directly into the bladder lumen, one then avoids all the side effects of the drug if given orally or by a parental route. For example, the patients’ hair does not fall out during intravesical mitomycin C administration.
A second positive is that the drug is not being metabolised and eliminated – the first-pass effect of the oral route is eliminated – so lower concentrations can be used to achieve the desired result, with physician knowing the drug is reaching its intended target.
Thirdly, the intravesical route allows, within reason, a predetermined time that the drug is in contact with the diseased tissues.
There are several downsides, including the catheter used to access the bladder lining. Although urologists blithely insert catheters on a daily basis, we sometimes forget it is an invasive process – one that is uncomfortable and sometimes even downright painful, despite the use of anaesthetic lubricants. For the patient, having a catheter inserted can also be an embarrassing or degrading experience.
Then there is the fact that the catheter breeches the immune system, and with the plethora of bacteria present in the genital region, from E.coli to Pseudomonas aeruginosa, this can result in urinary infections. These bacteria also induce biofilms which can retard drug access to the tumour.
Furthermore, the bladder has evolved to be impervious to most biological molecules. Urine is a toxic substance and nature designed a bladder epithelium that can not only stretch but is very good at keeping toxins out. Additionally, there is the surface glycose amino glycan layer (GAG) protecting the urothelium. Urologists know only too well the problems that arise in patients when this barrier becomes leaky.
The bladder’s primary role of urine storage does not stop during therapy. There is a continuous drainage of urine into the bladder from the kidneys which, on a time-based formula, gradually dilutes the instilled drug, so two hours is usually the maximum time period for effective therapy. One can, of course, increase successful therapy time by getting the patient to fluid-restrict.
Lastly, associated bladder pathology such as overactive bladder or interstitial cystitis can significantly shorten the time period available for effective dosing.
After the rough ride that thermotherapy experienced at the turn of the century, it would appear at least with bladder hyperthermia that the future is brighter, with an air of quiet excitement as more centres turn their gaze on this form of treatment. I have truly only glanced over this subject and for those of you who are more knowledgeable, I hope I did not disappoint too much. Hyperthermia technology is still in its infancy, with a large clinical hurdle to climb before it gains wide acceptance. The third and final article in this series will discuss all the important clinical papers documenting the efficacy and safety of chemo-hyperthermia, and how one day it may well replace BCG as a first-line therapy.
Abridged from The Development of Thermal Therapy for Bladder Cancer, Part 2, originally published in Urology, Uro-Oncology and Sexology Update (Summer Edition / Issue 2, 2022). Bibliography available here
