PNHの診断と治療の進歩

PNH understanding has advanced progressively and increasingly over the last two or three decades. From the 1980s it has been recognized that this is a disease related to complement activity on blood cells but it was originally thought of as a pure hemolytic anemia with some unusual thrombotic complications. In the 90s, some studies done by Professor Hillmen and his colleagues in the UK and some other investigators in France confirmed for us that this is a potentially lethal disease with a survival that was unacceptable for a nonmalignant disease with up to one third of patients dying within five years of diagnosis. However, complete understanding of this disease required the knowledge around the GPI anchor, its gene being mutated in patients with PNH and a complement shield being missing from red blood cells. Following that we saw the rapid development of therapeutics and further understanding of the disease. So we now are aware that this disease has, as its basis, intravascular hemolysis which is chronic, not intermittent, and that this chronic hemolysis with hemoglobin being released into the circulation results in nitric oxide depletion in the circulation which results in smooth muscle dystonia and is intimately involved in all of the complications of this disease, many of which are life-threatening and potentially fatal but all of which reduce the quality of life of these patients. So now we are in a situation of being able to block complement. We can understand that even patients who did not appear to have a poor quality of life actually now recognize that they did at the time. So the understanding is increasing dramatically and now we have additional science around the patients following the blockade of complement and understanding some concepts such as extra vascular hemolysis that might occur in these patients and some of the problems of patients with on an ongoing transfusion requirement. So the understanding of this disease continues to increase and I am sure will continue to increase as knowledge continues to accrue. Most importantly is understanding the pathophysiology of thrombosis, the most lethal complication of this disease and the many factors involved in this. Nitric oxide depletion is one but there are many others as well.

This has been an interesting meeting. ASH 2012 in Atlanta with regard to PNH, we have had several posters which have demonstrated a number of factors mainly around treatment and its long-term effects. So we have had an extension of the Kelly data presented which will demonstrate to us that long-term survival is indeed almost normalized in patients with PNH with effective complement blockade, although it will never be completely normalized because we also understand that blockade of complement does not affect the associated bone marrow disorders that many of these patients have and that these have with them a mortality risk as well. So patients with myeloid dysplastic syndrome, severe aplastic anemia, suffer the consequences of those diseases independent of control of their PNH. We also now have long-term safety data from the clinical trial population indicating that adverse events related to treatment continue to reduce in quantity with ongoing follow up, that the clinical benefit that was initially seen in the clinical trials and subsequent extension studies are maintained beyond five years, and we have also seen that the thrombosis risk in the study from the PNH registry reported by Dr. Socie continues, that is, that the almost complete abrogation of thromboembolic risk after complement blockade.

Along with the increasing understanding of the pathophysiology of PNH, the diagnosis of this disease has improved dramatically. In the 90s and for many decades before, the diagnostic tests were crude and lacked sensitivity, these were in vitro functional tests of complement sensitivity such as the Ham test, the acid lysis test, or the sucrose lysis test.
We now understand that what we're looking for is a deficiency of GPI anchor proteins which have been very well characterized on red cells, granulocytes, and monocytes, as well as platelets although that is not used as a diagnostic cell, and that flow cytometry is the benchmark, this is the gold standard that should be used at the moment. Now we have very sensitive testing for GPI deficient cells and can with high sensitivity flow cytometry detect populations of PNH cells at less than 0.1% of a cell population. The International Cytometry Society as well as the International PNH Interest Group have published guidelines on testing for PNH in patients who should be suspected of this condition and ongoing testing for patients who have an established diagnosis. This relates to flow cytometry of granulocytes as the primary target cell along with one other cell population which may be monocytes or red cells or both for more than one GPI anchored protein in each cell population using more than one reagent. This provides a complete picture of GPI anchor protein deficiency in the blood cells of patients suspected of having PNH. The testing is now standardized in many countries around the world and in Australia there has been quality assurance program by the College of Pathologists which has ensured that all laboratories doing PNH testing in our country are at a similar standard and provide a similar standard of reporting. The Cytometry Society has also provided a template for standardized reports which are clear, specific, quantitative, and easily understood by a practicing hematologist so that there is no doubt that a clone has been identified, what size it is, and in what cell population. While this may be regarded as a hemolytic disease it is the characteristics of the non-red cells that are most important in primary diagnosis as many factors can interfere with quantization of red blood cells. In particular the most sensitive cells to hemolysis, to complement activity in PNH, disappear because of that hemolysis and so are not counted in flow cytology. They also may be diluted by red cell transfusion. So the red cell may be useful to look at after treatment has commenced but is not very helpful at diagnosis very often. So who should be tested has also been addressed by these societies, and there are a number of patient groups in whom it is important to establish a diagnosis of PNH if it exists. Patients with bone marrow failure syndromes such as aplastic anemia and low grade myeloid dysplastic syndromes not infrequently have a PNH clone associated with those diagnoses. Sometimes it is very small, but it is actually very important to establish that they are present in order to monitor them, as with treatment of these underlying disorders the PNH clone may become dominant, and PNH requiring treatment may evolve as a diagnosis. In addition thrombosis is the most important side effects of chronic intravascular hemolysis, it is certainly the most lethal complication of PNH. When patients present with thrombosis that is not clearly related to an underlying cause, is in an unusual location, perhaps in the arterial tree in a young person with no risk factors, then this is a diagnosis that needs to be considered, as PNH is actually the most highly thrombogenic state that we know of, certainly more than any of the inherited thrombophilic states. So it does need to be considered, although with the frequency of thromboses in the general population one certainly can’t test every patient with a deep venous thrombosis of the calf veins for example. In addition, patients who are cytopenic with no known cause should be investigated, and also patients who may have some mild cytopenias that otherwise would be ignored but have recurrent iron deficiency with no apparent cause may be a good target population These patients may have low-grade hemoglobinuria which is not detected macroscopically but have a chronic iron loss that is otherwise unsuspected. Hemoglobinuria is an obvious indication, as is Coombs negative hemolytic anemia, but hemoglobinuria can clinically be confused with hematuria depending on the specialist to which such a patient is referred, and it is not infrequent that they have urologic investigations for sometime, before it is recognized that this is hemoglobinuria rather than hematuria. So there are wide variety of patients for whom testing is potentially indicated but with clinical understanding of the disease one can target testing and make it appropriate.

The diagnosis of PNH is becoming more accurate and more widespread in understanding by hematology practitioners. Hematologists now understand that this disease has treatment and the diagnosis is now more than of academic interest particularly in patients who don’t have overt hemolysis. With a standardized diagnostic system amongst the major laboratories in Australia it has now become apparent that doctors are thinking about this diagnosis more frequently in the appropriate patient groups and sometimes very cleverly making diagnoses that result in appropriate referrals for treatment. These days the number of new patients that we are seeing in the country is a steady flow at about the expected incidence rate and the diagnostic testing we think uniformly around Australia is very good. I think that most of the rest of the developed world, certainly where treatment is available to the patients, has followed suit and we continue to see improvements elsewhere in the world as well. What is more important is in fact that the variability in clinical presentation of patients with PNH is now being increasingly recognized because the diagnosis is being sought more often. So patients with very complex clinical situations with unusual bone marrow failure syndromes are now being seen which become a therapeutic challenge as they may have two or three life-threatening diagnoses coexisting with their PNH, and this provides a great deal of clinical discussion and thought about what the appropriate therapeutic direction is. I think it is fair to say that our experience to this date has shown us that even in patients in whom the other bone marrow disorders are difficult to sort out, while waiting to get those organized, if a patient has true hemolytic PNH or PNH that looks like it requires complement blockade, that that should proceed, that one first controls the uncontrolled action of complement in PNH patients and then sorts out the other disorders on their own.

We now have a monoclonal antibody, eculizumab, that is capable of blocking complement activity in almost all patients with paroxysmal nocturnal hemoglobinuria and this has been transformative in the management of these patients. We now understand which patients require treatment, it varies a little bit depending on funding arrangements in different countries and different regions but there are fundamental principles relating to a core group of patients that do require treatment with eculizumab. We now know that such treatment is required lifelong without interruption, that it is generally extremely well tolerated, that cessation is rarely required for toxicity reasons, and that patient's quality of life and survival is transformed by ongoing treatment. We also recognize that blockade of the part of the complement cascade at C5 that this antibody causes may result in some secondary effects that require some additional thought.

The development of complement blockade with eculizumab has been guided by a series of excellent clinical trials, including a very bravely constructed placebo controlled clinical trial, the TRIUMPH study, which very clearly demonstrated the rapid effectiveness and ongoing effectiveness of eculizumab compared to saline infusions, placebo infusions. That study combined with the prior pilot study and subsequently the SHEPHERD trial which was an open label study in a broader group of patients including those with some evidence of bone marrow failure as well, allowed us to understand that almost all patients with PNH meeting certain criteria were capable of significant improvement with complement blockade. The Japanese AEGIS study also taught us that response in Japanese patients was essentially the same as those in the other global studies, although it did highlight the very small incidence of gene mutations in part of the complement system that was specific to the Japanese population as far as we know so far and results in some resistance to treatment in that population.
The trials were further informed by the extension study into which these patients and the patients in the original pilot study of Hillmen were treated, and that confirmed at least for the first 18 months after onset of treatment, the ongoing stability of these patients. Then we have ongoing examination of these patients’ outcomes both at the individual trial level but also through the global PNH Registry, which is proving an increasing resource for understanding the effectiveness of the treatment.

So while all patients treated with eculizumab have a reduction in transfusion requirement, improvement in fatigue, and other quality-of-life parameters, and improvement in anemia, not all patients become completely transfusion independent and many patients have an ongoing transfusion requirement. This may be because of a coexistent bone marrow failure syndrome but sometimes it is related to the development of extravascular hemolysis, a complication that is being considered more frequently now in patients. This does not represent a failure of treatment with eculizumab, but does represent a new clinical problem that requires attention. So these patients who have an ongoing transfusion requirement without urea iron loss are at risk of iron accumulation and toxicity from iron overload, and that in itself is a reason to consider some form of intervention if possible to prevent the extravascular hemolysis and reduce transfusion requirement. This may take many forms. There may be adjunctive therapies such as iron chelation therapy to reduce the iron overload in these patients and some other clinical interventions may be required which require some thought. So splenectomy has been considered by the Italians and our own group in some patients in this situation, and there are some ongoing studies with inhibitors of a more proximal part of the complement cascade, C3, which may be involved in this extravascular hemolysis in some of these patients. So this is an area that will be looked at very closely in an ongoing way. But in general the transformation of the lives of patients by therapeutic intervention since the introduction of eculizumab in the clinical trials has been wonderful.

In Australia we have a unique system of funding expensive drugs for life-saving indications. There is a program of the federal government linked to but not directly part of the usual pharmaceutical funding scheme. This is called the Life Saving Drugs Program. PNH is the only condition in that list so far which is not an enzyme deficiency but has a number of characteristics in common with those other disorders in that it has a life-threatening component, there is therapy available which is demonstrably life-saving, and that it affects a rare group of patients, a very rare group of patients, and that the cost of this medication is beyond the reach of any individual. And in fact drugs are able to be listed on this program only if they meet all of the criteria of the usual drug funding scheme but are not cost-effective in a pharmacoeconomic sense based on criteria established by our drug regulators. The management of this is quite unique in the world in that the physician looking after the patient only has the ability to apply for therapy for his patient, and if approved then his only power is in fact to write the orders for administration and to provide data to a central committee to ensure that compliance is being maintained and that treatment is appropriate. There is a disease advisory committee for PNH that sits on an ongoing basis through the year but formally twice a year, which considers every application from a physician, decides if they meet our criteria, makes a recommendation to the government about funding, who will then decide whether or not the patient should be funded. And if they do, it is the Life Saving Drugs Program that orders the drug from the company, directly supplies it to the pharmacy of the hospital in which the patient will be treated, and monitors its use so that they know every dose that has been dispensed and know when it has been dispensed so they ensure that the recommendations of the committee have been met and ask questions in real time if they haven’t been met. This is a very efficient system. It takes some control away from physicians which is an unusual thing for, particularly for hematologists who are used to having a great deal of control over what they do with their patients but it is very good for patient monitoring and outcome. Data provision is a very important part of that and if a physician does not provide the appropriate data to confirm that the patients are being treated in an appropriate way then a series of warning letters will be written to the physician and eventually to the patient indicating that compliance has not been maintained. So there is a possibility of withdrawing treatment from a patient if the physician and the patient are not complying with the prescribed regimen. It has worked very well. The committee is formed of six hematologists with an interest in and expertise in PNH around the country. It is a very coherent committee and all of the deliberations are recorded and documented in minutes of every meeting. So there is an ongoing trial. This is actually a very good data resource for us as well and will be in the future.