RESUMEN
Controlled human infection studies (CHIs) involve the intentional infection of human subjects for a scientific aim. Though some past challenge trials have involved serious ethical abuses, in the last few decades, CHIs have had a strong track record of safety. Despite increased attention to the ethics of CHIs during the COVID-19 pandemic, CHIs remain controversial, and there has been no in-depth treatment of CHIs through the lens of virtue ethics. In this article, we argue that virtue theory can be helpful for addressing CHIs that present a constellation of controversial, unresolved, and/or under-regulated ethical issues. We begin with some brief background on virtue ethics. We then substantiate our claim that some CHIs raise a constellation of ethical issues that are unresolved in the ethics literature and/or lack adequate regulatory guidance by demonstrating that CHIs can present indeterminate social value, risks to third parties, limitations on the right to withdraw from research, and questions about the upper limit of allowable risk. We argue that the presence of a virtuous investigator, with virtues such as prudence, compassion, and integrity, is especially important when these unresolved research ethics issues arise, which is the case for certain types of controlled human infection studies. We use the historical example of Walter Reed and the Yellow Fever Commission to illustrate this claim, and we also highlight some contemporary examples. We end by sketching some practical implications of our view, such as ensuring that investigators with experience running CHIs are involved in novel CHI models.
Asunto(s)
COVID-19 , SARS-CoV-2 , Virtudes , Humanos , Teoría Ética , Ética en Investigación , Pandemias/ética , Ensayos Clínicos Controlados como Asunto/ética , Valores SocialesRESUMEN
ensaio clínico de pacientes únicos (ECPU/ensaio n-de-1) consiste na observação sistemática de condutas terapêuticas adotadas para otimizar o restabelecimento da saúde em um único paciente, com múltiplos cruzamentos ao longo do tratamento, podendo ter adicionalmente propósito de pesquisa clínica. Foi proposto há décadas e tem sido mais utilizado nas áreas de psicologia clínica, recebendo maior atenção em estudos médicos nos últimos anos. Embora seja considerado como o tipo de estudo com maior força para tomada de decisões terapêuticas, ainda são escassas as publicações sobre o seu emprego em medicina. Este artigo aborda as possibilidades dos ECPUs na avaliação dos resultados clínicos da homeopatia, explorando seus aspectos metodológicos, éticos e educacionais característicos em comparação aos ensaios clínicos randomizados tradicionais. Em pesquisa clínica, diferentemente dos ensaios convencionais, os ECPUs permitem a participação mais direta do paciente na escolha dos procedimentos e acompanhamento dos resultados, com possibilidade de alterações imediatas e sem que seja necessária sua exclusão do estudo, além de implicações de ordem econômica, política e ética. Podem ser utilizados no teste de medicamentos usados de modo off label, sem as restrições impostas à inclusão de pacientes vulneráveis nos estudos clínicos habituais, com excessiva artificialização no delineamento experimental. Poderiam ser ainda adotados nas diversas fases de teste clínico dos medicamentos, reduzindo a exposição de grande número de participantes aos riscos da pesquisa e baixa margem de extrapolação clínica dos resultados ao conjunto da população. Em homeopatia, podem ajudar a aprimorar o conhecimento dos medicamentos já em uso ou a melhor detectar os efeitos de novas substâncias testadas em ensaios patogenéticos homeopáticos. Em função do seu propósito principal de otimização do tratamento individual e do alinhamento com os princípios éticos da autonomia e beneficência associados à prática da medicina centrada-no-paciente ou de precisão - podem ser desenvolvidos em conjunto com o paciente e familiares, sem a obrigatoriedade de aprovação prévia por Comissões de Ética Médica ou Comitês de Ética em Pesquisa.
The single-patient clinical trial (n-of-1 trial) is primarily designed to systematically observe outcomes from different therapeutic options to optimize the restoration of health in a single patient, with multiple crossovers throughout the treatment. They may additionally have a clinical research purpose. They have been proposed for decades and were mostly used in clinical psychology, receiving greater attention in medical studies in recent years. Although it is considered the type of study with the greatest strength for therapeutic decision-making, there are still few publications with its application in medicine. This article discusses the possibilities of single-patient clinical trials in assessing homeopathy outcomes, exploring their characteristic methodological, educational and ethical aspects compared to traditional randomized clinical trials. In clinical research, unlike conventional trials, single-patient clinical trials allow for more direct patient participation in choosing procedures and monitoring results, with the possibility of immediate changes without the need for their exclusion from the study, in addition to economic, political and ethical implications. They can be used in testing off-label drugs without the restrictions imposed on the inclusion of vulnerable patients in usual clinical studies, with excessive artificiality in the experimental design. They could also be adopted in the various clinical trial phases of drugs, reducing the exposure of many participants to the risks of research and low margin of clinical extrapolation of the results to the entire population. In homeopathy, they can help refine the knowledge of medications already in use or better detect the effects of new substances tested in homeopathic pathogenetic trials. Due to their purpose of optimizing individual treatment and alignment with the principles of patient-centered or precision medicine they can be developed jointly with the patient and her family without the mandatory prior approval by Medical Ethics Committees or Research Ethics Committees.
Asunto(s)
Humanos , Clínica Homeopática , Conductas Terapéuticas Homeopáticas/normas , Ensayos Clínicos Controlados como Asunto/ética , Ética Médica , Medicina de PrecisiónRESUMEN
The current version of the Declaration of Helsinki states that 'the benefits, risks, burdens and effectiveness of a new intervention must be tested against those of the best current proven intervention(s) '. This wording implies that it is acceptable for patients to be assigned to receive an unproven new intervention and to be denied a best current proven intervention. We assert that patients being invited to participate in controlled trials cannot, ethically, be expected to forego proven beneficial forms of care. Patients being treated in controlled trials should not knowingly be disadvantaged compared with similar patients being treated in usual clinical care, where they have access to beneficial care. In this article, we have tried to separate for discussion 'the withholding of effective care from trial participants', 'informed consent to treatment', 'blinding' and 'use of placebos'.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/ética , Ensayos Clínicos Controlados como Asunto/normas , Placebos/uso terapéutico , Nivel de Atención , Experimentación Humana Terapéutica/ética , Privación de Tratamiento/ética , Método Doble Ciego , Declaración de Helsinki , Humanos , Consentimiento InformadoAsunto(s)
Antivirales , Ensayos Clínicos Controlados como Asunto , Medicina Basada en la Evidencia , Placebos , Humanos , Antivirales/administración & dosificación , Ensayos Clínicos Controlados como Asunto/ética , Ensayos Clínicos Controlados como Asunto/normas , COVID-19/mortalidad , Tratamiento Farmacológico de COVID-19 , Medicina Basada en la Evidencia/ética , Medicina Basada en la Evidencia/normas , Placebos/administración & dosificación , Placebos/normas , Estados Unidos , United States Food and Drug Administration/normasRESUMEN
The stated purpose of sham or placebo surgery is to enable the implementation of surgical placebo-controlled trials (SPTs) for evaluating the safety and efficacy of surgical interventions. Exposing the participants to the burdens and harms of sham surgery has been justified on the grounds of the absolute necessity for controlling large placebo effects and observer bias, assumed to be associated with surgical procedures. In the present review, we argue that evidence obtained from SPTs of cellular therapies for the treatment of Parkinson's disease (PD) has failed to demonstrate either large and consistent placebo effects or decisive methodological advantages for relying on sham surgical controls. We outline several alternative assessment strategies and designs available to establish the efficacy of cellular therapies. It is concluded that the evidence evaluated in the present analysis indicated that use of sham surgery in the context of developing novel surgical procedures for PD is not necessary, and therefore, unethical under a utilitarian model.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/ética , Evaluación de Procesos y Resultados en Atención de Salud , Enfermedad de Parkinson/cirugía , Efecto Placebo , Trasplante de Células Madre , Medicina Basada en la Evidencia , Humanos , PlacebosRESUMEN
On March 6, 2019, a workshop was held as part of a larger public consultation exercise to evaluate the perceptions of participants from diverse backgrounds of studies involving Controlled Human Infection Models (CHIMs) (1,2) in India, through three specific case scenarios. This workshop was organised by the Health and Humanities Division of the St. John's Research Institute, Bangalore with funding from the Translational Health Science and Technology Institute (TSHTI), Faridabad (www.thsti.res.in), an autonomous institute of the Department of Biotechnology, Government of India This was an on-going effort of the Division to bring public discourse centre stage in the discussion on the use, ethics and regulations related to CHIM studies, and the introduction of such studies in India. Participants included epidemiologists, community/public health experts, microbiologists, infectious disease specialists, basic and translational scientists, ethicists, journalists and lawyers.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/ética , Ensayos Clínicos Controlados como Asunto/legislación & jurisprudencia , Ensayos Clínicos Controlados como Asunto/psicología , Experimentación Humana/ética , Experimentación Humana/legislación & jurisprudencia , Academias e Institutos , Fiebre Chikungunya/prevención & control , Humanos , India/epidemiología , Malaria/prevención & control , Salud Pública , Opinión Pública , Fiebre Tifoidea/prevención & controlRESUMEN
The Controlled Human Infection Model or CHIM, sometimes described as a human challenge study, is a relatively specialised medical research technique. Researchers infect healthy participants with a weakened strain of a pathogen in a controlled setting, in order to learn more about the infection and the disease, or to develop new vaccines for that disease. Unlike in other human clinical trials, where participants face a risk of harm because of, for example, the drug's side effects, healthy participants in CHIM trials are deliberately harmed through infection - contrary to every principle and guideline of medical practice and research.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/ética , Ensayos Clínicos Controlados como Asunto/métodos , Ética en Investigación , Experimentación Humana/ética , Sujetos de Investigación , Humanos , India , Control de Infecciones , Infecciones/terapia , Salud Pública , Opinión Pública , Participación de los InteresadosRESUMEN
This paper considers the ethics of placebo-controlled trials in developing countries, where a treatment already exists but is not available due to the low local standard of care. Such trials would not be permitted in more developed nations where a higher standard of care is available. I argue that there are moral intuitions against such trials, but a further intuition that if the trials were aimed at producing treatment options for the developing world, that would be more permissible than if the trials were designed with the benefit of rich world people in mind. An approach based upon GA Cohen's work on interpersonal justifiability is suggested to allow us to explain these intuitions. Cohen's framework shows that these trials are ethically problematic because the inequality in healthcare provision between developing and developed nations that allows them to take place is at least partly the pharmaceutical corporations' fault. Following Cohen's argument, this means the trials are non-comprehensively justified. This allows for a more complete explanation of our intuitions than to consider such trials as cases of exploitation, because intuitions on the ethicacy of research can vary even when the exploitation relation remains the same. It is then established that there is good empirical evidence to believe that pharmaceutical corporations do fail the interpersonal justifiability test. The policy implications of this judgement are then considered, and it is suggested that the framework might be equally applicable to examining the permissibility of research conducted on vulnerable people within more developed nations.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/ética , Países en Desarrollo , Análisis Ético , Sujetos de Investigación , Nivel de Atención , Industria Farmacéutica/ética , Industria Farmacéutica/normas , Humanos , América Latina , Condición Moral , Placebos , Estados UnidosRESUMEN
BACKGROUND: In October 2013, the Declaration of Helsinki was revised a seventh time in its 50 year history. While it is the most widely accepted set of ethical principles for the protection of patients participating in medical research, the Declaration of Helsinki has also been subject of constant controversy. In particular, its paragraph on the use of placebo controls in clinical trials divides the research community into active-control and placebo orthodox proponents, both continuously demanding revisions of the Declaration of Helsinki in favour of their position. The goal of the present project is to compare the mainly theoretical controversy with regulatory implementation. METHODS: We distributed a questionnaire to national drug regulatory authorities from different countries to collect information on the authorities' respective approaches to interpretation and implementation of the Declarations' placebo paragraph in the conduct of medical research. RESULTS: Our findings suggest that the majority of drug regulatory authorities have established a practice of a middle ground, allowing placebo controls in some instances. Various interpretations of "serious harm" and "methodological reasons" are proposed as well as safeguards to avoid abuse of the option to use placebo-controls. CONCLUSION: Leaving the placebo paragraph open to various interpretation is a result of the Declaration of Helsinki's character as a guidance document. With the current version controversy will continue. The Declaration should be continued to be strengthened to enforce the appreciation of conducting medical research with the highest ethical standard.
Asunto(s)
Investigación Biomédica/ética , Comprensión , Disentimientos y Disputas , Declaración de Helsinki , Lenguaje , Efecto Placebo , Placebos , Ensayos Clínicos Controlados como Asunto/ética , Control de Medicamentos y Narcóticos , Ética en Investigación , Regulación Gubernamental , Humanos , Ética Basada en Principios , Encuestas y Cuestionarios , TraduccionesRESUMEN
There are many difficulties in undertaking independent clinical research without support from the pharmaceutical industry. In this retrospective observational study, some design characteristics, the clinical trial public register and the publication rate of noncommercial clinical trials were compared to those of commercial clinical trials. A total of 809 applications of drug-evaluation clinical trials were submitted from May 2004 to May 2009 to the research ethics committee of a tertiary hospital, and 16.3% of trials were noncommercial. They were mainly phase IV, multicentre national, and unmasked controlled trials, compared to the commercial trials that were mainly phase II or III, multicentre international, and double-blind masked trials. The commercial trials were registered and published more often than noncommercial trials. More funding for noncommercial research is still needed. The results of the research, commercial or noncommercial, should be disseminated in order not to compromise either its scientific or its social value.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/economía , Ensayos Clínicos Controlados como Asunto/métodos , Industria Farmacéutica/economía , Comités de Ética en Investigación , Proyectos de Investigación , Apoyo a la Investigación como Asunto/economía , Ensayos Clínicos Fase II como Asunto/economía , Ensayos Clínicos Fase II como Asunto/métodos , Ensayos Clínicos Fase III como Asunto/economía , Ensayos Clínicos Fase III como Asunto/métodos , Ensayos Clínicos Fase IV como Asunto/economía , Ensayos Clínicos Fase IV como Asunto/métodos , Ensayos Clínicos Controlados como Asunto/ética , Industria Farmacéutica/ética , Humanos , Estudios Multicéntricos como Asunto/economía , Estudios Multicéntricos como Asunto/métodos , Sistema de Registros , Apoyo a la Investigación como Asunto/ética , Estudios RetrospectivosAsunto(s)
Angina Estable , Ensayos Clínicos Controlados como Asunto , Evaluación de Resultado en la Atención de Salud/métodos , Intervención Coronaria Percutánea/métodos , Placebos , Angina Estable/diagnóstico , Angina Estable/fisiopatología , Angina Estable/psicología , Angina Estable/cirugía , Ensayos Clínicos Controlados como Asunto/ética , Ensayos Clínicos Controlados como Asunto/métodos , Ensayos Clínicos Controlados como Asunto/psicología , Humanos , Efecto Placebo , Periodo Posoperatorio , Medición de Riesgo , Evaluación de Síntomas/métodosRESUMEN
Randomized placebo-controlled clinical trials have been considered the most rigorous method of evaluating the efficacy of novel treatment interventions. The first effective disease-modifying therapies (DMTs) for relapsing-remitting multiple sclerosis (RRMS) were approved in the 1990s after a number of pivotal placebo-controlled trials. Since then, the ethics of the continued use of placebo in clinical trials of new DMTs for RRMS has been the subject of repeated policy statements and recommendations by international committees. As further data have accumulated demonstrating a reduction in long-term morbidity and mortality with early initiation of DMT, a growing consensus has emerged that further inclusion of placebo arms in clinical trials of novel RRMS therapies is no longer ethical.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/ética , Esclerosis Múltiple Recurrente-Remitente/tratamiento farmacológico , HumanosRESUMEN
BACKGROUND: Improper practices and unprofessional conduct in clinical research have been shown to waste a significant portion of healthcare funds and harm public health. OBJECTIVES: Our objective was to evaluate the effectiveness of educational or policy interventions in research integrity or responsible conduct of research on the behaviour and attitudes of researchers in health and other research areas. SEARCH METHODS: We searched the CENTRAL, MEDLINE, LILACS and CINAHL health research bibliographical databases, as well as the Academic Search Complete, AGRICOLA, GeoRef, PsycINFO, ERIC, SCOPUS and Web of Science databases. We performed the last search on 15 April 2015 and the search was limited to articles published between 1990 and 2014, inclusive. We also searched conference proceedings and abstracts from research integrity conferences and specialized websites. We handsearched 14 journals that regularly publish research integrity research. SELECTION CRITERIA: We included studies that measured the effects of one or more interventions, i.e. any direct or indirect procedure that may have an impact on research integrity and responsible conduct of research in its broadest sense, where participants were any stakeholders in research and publication processes, from students to policy makers. We included randomized and non-randomized controlled trials, such as controlled before-and-after studies, with comparisons of outcomes in the intervention versus non-intervention group or before versus after the intervention. Studies without a control group were not included in the review. DATA COLLECTION AND ANALYSIS: We used the standard methodological procedures expected by Cochrane. To assess the risk of bias in non-randomized studies, we used a modified Cochrane tool, in which we used four out of six original domains (blinding, incomplete outcome data, selective outcome reporting, other sources of bias) and two additional domains (comparability of groups and confounding factors). We categorized our primary outcome into the following levels: 1) organizational change attributable to intervention, 2) behavioural change, 3) acquisition of knowledge/skills and 4) modification of attitudes/perceptions. The secondary outcome was participants' reaction to the intervention. MAIN RESULTS: Thirty-one studies involving 9571 participants, described in 33 articles, met the inclusion criteria. All were published in English. Fifteen studies were randomized controlled trials, nine were controlled before-and-after studies, four were non-equivalent controlled studies with a historical control, one was a non-equivalent controlled study with a post-test only and two were non-equivalent controlled studies with pre- and post-test findings for the intervention group and post-test for the control group. Twenty-one studies assessed the effects of interventions related to plagiarism and 10 studies assessed interventions in research integrity/ethics. Participants included undergraduates, postgraduates and academics from a range of research disciplines and countries, and the studies assessed different types of outcomes.We judged most of the included randomized controlled trials to have a high risk of bias in at least one of the assessed domains, and in the case of non-randomized trials there were no attempts to alleviate the potential biases inherent in the non-randomized designs.We identified a range of interventions aimed at reducing research misconduct. Most interventions involved some kind of training, but methods and content varied greatly and included face-to-face and online lectures, interactive online modules, discussion groups, homework and practical exercises. Most studies did not use standardized or validated outcome measures and it was impossible to synthesize findings from studies with such diverse interventions, outcomes and participants. Overall, there is very low quality evidence that various methods of training in research integrity had some effects on participants' attitudes to ethical issues but minimal (or short-lived) effects on their knowledge. Training about plagiarism and paraphrasing had varying effects on participants' attitudes towards plagiarism and their confidence in avoiding it, but training that included practical exercises appeared to be more effective. Training on plagiarism had inconsistent effects on participants' knowledge about and ability to recognize plagiarism. Active training, particularly if it involved practical exercises or use of text-matching software, generally decreased the occurrence of plagiarism although results were not consistent. The design of a journal's author contribution form affected the truthfulness of information supplied about individuals' contributions and the proportion of listed contributors who met authorship criteria. We identified no studies testing interventions for outcomes at the organizational level. The numbers of events and the magnitude of intervention effects were generally small, so the evidence is likely to be imprecise. No adverse effects were reported. AUTHORS' CONCLUSIONS: The evidence base relating to interventions to improve research integrity is incomplete and the studies that have been done are heterogeneous, inappropriate for meta-analyses and their applicability to other settings and population is uncertain. Many studies had a high risk of bias because of the choice of study design and interventions were often inadequately reported. Even when randomized designs were used, findings were difficult to generalize. Due to the very low quality of evidence, the effects of training in responsible conduct of research on reducing research misconduct are uncertain. Low quality evidence indicates that training about plagiarism, especially if it involves practical exercises and use of text-matching software, may reduce the occurrence of plagiarism.
Asunto(s)
Investigación Biomédica/ética , Plagio , Investigadores/ética , Mala Conducta Científica/ética , Actitud , Estudios Controlados Antes y Después/ética , Estudios Controlados Antes y Después/normas , Ensayos Clínicos Controlados como Asunto/ética , Ensayos Clínicos Controlados como Asunto/normas , Humanos , Edición/ética , Edición/normas , Ensayos Clínicos Controlados Aleatorios como Asunto/ética , Ensayos Clínicos Controlados Aleatorios como Asunto/normas , Investigadores/normasAsunto(s)
Trastorno del Espectro Autista/tratamiento farmacológico , Trastornos de los Cromosomas/tratamiento farmacológico , Drogas en Investigación/uso terapéutico , Ética en Investigación , Factor I del Crecimiento Similar a la Insulina/uso terapéutico , Selección de Paciente/ética , Clase Social , Justicia Social , Trastorno del Espectro Autista/economía , Niño , Deleción Cromosómica , Trastornos de los Cromosomas/economía , Cromosomas Humanos Par 22 , Ensayos Clínicos Fase II como Asunto/ética , Ensayos Clínicos Controlados como Asunto/ética , Estudios Cruzados , Método Doble Ciego , Drogas en Investigación/economía , Geografía , Humanos , Factor I del Crecimiento Similar a la Insulina/economíaRESUMEN
Currently, there are no approved treatments for NMO. All therapeutic studies in NMO have been either small, retrospective case series or uncontrolled prospective studies. Such studies are susceptible to inherent biases. As a consequence, conclusions regarding efficacy and safety from these studies may be erroneous. The optimal method for assessing therapeutic efficacy is the prospective, controlled trial with random treatment assignment that has the potential to control for multiple sources of bias. There is a significant unmet need for well-designed clinical trials in NMO. Successfully conducted, well-controlled NMO trials that show proof of benefit will lead to regulatory approval and subsequent acceptance by payers resulting in broad therapeutic availability. The most direct method to prove efficacy is to compare an active treatment vs. no treatment or placebo control. However, because of the devastating nature of the disease some clinicians are reluctant to expose potential study patients to the risk of no treatment. The primary ethical concern in the case of placebo-control in NMO clinical trials rests on the relative merits of answering the scientific question regarding efficacy compared to the relative risk of exposure to harm in the placebo-control group. This article outlines the case for clinical equipoise in NMO by addressing the uncertainty regarding the relative scientific and clinical merits of current empirically used treatments and showing that a placebo arm is consistent with competent medical care. Because no currently available treatment has proven benefit, and because all therapies are known to potentially cause harm, placebo-control is not only ethical but is in some ways preferable to active comparator or add-on study designs. Without well-designed, placebo-controlled trials, NMO patients may not have access to new treatments and will never know whether the therapies that they may be currently taking have risk to benefit profiles that clearly favor their use.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/ética , Neuromielitis Óptica/tratamiento farmacológico , Humanos , Factores Inmunológicos/uso terapéuticoRESUMEN
This paper addresses the ethical acceptability of a proposed placebo controlled trial of a new intervention as a possible relapse prevention treatment for Neuromyelitis Optica (NMO). In the analysis of this controversial ethical issue, the author points out significant factors that are often overlooked or ignored, such as the life-long implications for study participants and others living with the disease, and also addresses commonly noted issues, such as vulnerability, benefits, harms, and justice that always require attention in research review.
Asunto(s)
Ensayos Clínicos Controlados como Asunto/ética , Neuromielitis Óptica/terapia , Prevención Secundaria/ética , Humanos , Neuromielitis Óptica/diagnóstico , Neuromielitis Óptica/epidemiologíaRESUMEN
Permissibility of placebo controls in psychiatric research is raising everlasting controversies. The main ethical issue remains: whether, when, under what conditions, and to what extent is it justifiable to disregard subject's present (best) interest for the presumably "greater" ones. In relation to this main ethical concern, two distinct arguments arose: proponents of placebo controls trials (placebo ortxodoxy) and proponents of active controls trials (active-control orthodoxy). More recently, in new ethical guidelines, Declaration of Helsinki and International Ethical Guidelines for Biomedical Research Involving Human Subjects, a "middle way" approach was formulated, acceptable to both sides of the argument, saying placebo controls can be justified under certain conditions: when and only when, they firstly present undisputed methodological reasoning, and secondly, fulfill certain ethical considerations - mainly regarding the permissibility of accompanied risks. These ethical evaluations are inevitably contextual and evoke the need for the principle of proportionality. In scope of recent findings of substantial and progressively increasing placebo response in psychiatric research, contextual factors are identified and both theoretical and practical challenges are discussed.