The relative tolerance of children and adults to anticancer drugs

Lack of value of juvenile animal toxicity studies for supporting the safety of pediatric oncology phase I trials

Toxicity studies in juvenile animals (JAS) are sometimes performed to support clinical trials in pediatric oncology patients, and there are differing conclusions on the value of JAS for pediatric drug development. This manuscript provides a review of the pediatric clinical data for 25 molecularly-targeted and 4 biologic anticancer therapeutics. Other publications that evaluated the value of JAS in pediatric drug development focus on differences in toxicity between juvenile animals and adult animals. The present paper examines pediatric-specific clinical findings to focus on dose setting in pediatric oncology patients and safety monitoring in terms of the potential value of JAS. Our assessment demonstrates that pediatric starting doses were safe for all 29 therapeutics examined in that no life-threatening toxicities occurred in the first cohort, and overall the ratio of the pediatric maximum tolerated dose (MTD) to the recommended adult dose was close to 1. In addition, the 4 serious adverse events (SAEs) that weren't detectable with standard monitoring plans for pediatric oncology trials would not have been detectable in a standard JAS. This review demonstrates that safe starting doses in pediatric oncology patients for these therapeutics could have been solely based on adult doses without any knowledge of findings in JAS.

Dispelling urban myths about default uncertainty factors in chemical risk assessment--sufficient protection against mixture effects?

Assessing the detrimental health effects of chemicals requires the extrapolation of experimental data in animals to human populations. This is achieved by applying a default uncertainty factor of 100 to doses not found to be associated with observable effects in laboratory animals. It is commonly assumed that the toxicokinetic and toxicodynamic sub-components of this default uncertainty factor represent worst-case scenarios and that the multiplication of those components yields conservative estimates of safe levels for humans. It is sometimes claimed that this conservatism also offers adequate protection from mixture effects. By analysing the evolution of uncertainty factors from a historical perspective, we expose that the default factor and its sub-components are intended to represent adequate rather than worst-case scenarios. The intention of using assessment factors for mixture effects was abandoned thirty years ago. It is also often ignored that the conservatism (or otherwise) of uncertainty factors can only be considered in relation to a defined level of protection. A protection equivalent to an effect magnitude of 0.001-0.0001% over background incidence is generally considered acceptable. However, it is impossible to say whether this level of protection is in fact realised with the tolerable doses that are derived by employing uncertainty factors. Accordingly, it is difficult to assess whether uncertainty factors overestimate or underestimate the sensitivity differences in human populations. It is also often not appreciated that the outcome of probabilistic approaches to the multiplication of sub-factors is dependent on the choice of probability distributions. Therefore, the idea that default uncertainty factors are overly conservative worst-case scenarios which can account both for the lack of statistical power in animal experiments and protect against potential mixture effects is ill-founded. We contend that precautionary regulation should provide an incentive to generate better data and recommend adopting a pragmatic, but scientifically better founded approach to mixture risk assessment.

United States Voluntary Children's Chemical Evaluation Program (VCCEP) risk assessment for children exposed to benzene

As part of the Voluntary Children's Chemical Evaluation Program (VCCEP) program, a risk assessment was performed to evaluate the risks to children from environmental benzene exposures. This paper summarizes this risk assessment. Risk was characterized using two distinct methods: USEPA's default type of risk assessment, which used the Reference Dose (RfD) and Cancer Slope Factor (CSF) to characterize non-cancer and cancer risks, as well as a Margin of Safety (MOS) approach that utilized a point of departure (POD). The exposures for most scenarios evaluated in this VCCEP risk assessment are lower than both the cancer and non-cancer PODs by several orders of magnitude, indicating a large MOS and corresponding low potential for toxicity at these exposures. The highest benzene exposures likely experienced by children, associated with the lowest MOS, are from cigarette smoke. In addition, the potential for age-related differences in the sensitivity towards benzene-induced toxicity was investigated. In general, this risk assessment does not indicate that children are likely to be at a elevated risk of AML or hematopoietic toxicity associated with environmental exposures to benzene.

Proposal of new uncertainty factor application to derive tolerable daily intake

We propose new uncertainty factors (UFs) and a new subdivision of default factors in chemical risk assessment using a probabilistic approach based on the latest applicable information. Rounded values of 150 for mice, 100 for hamsters and rats, and 40 for rabbits, monkeys and dogs for inter- and intra-species differences (UF(AH)) were derived from the probabilistic combination of two log-normal distributions. Further calculation of additional UFs when chronic data (UF(S)) or NOAEL (UF(L)) are lacking was conducted using available log-normal distribution information. The alternative UF(S) and UF(L) values of 4 are considered to be appropriate for both cases where data are lacking. The default contributions of inter-species difference (UF(A)) and intra-species difference (UF(H)) to the UF(AH) of 100 for hamsters and rats as an example are considered to be 25 and 4, respectively. The UF(A) of 25 was subdivided into 25(0.6) (i.e., 7.0) for pharmacokinetics (PK) (UF(A,PK)) and 25(0.4) (i.e., 3.6) for pharmacodynamics (PD) (UF(A,PD)), and the UF(H) of 4 was evenly subdivided into 4(0.5) (i.e., 2) (UF(H,PK) and UF(H,PD)), to account for chemical-specific difference data between humans and laboratory animals for PK and/or PD. These default UFs, which come from actual experimental data, may be more appropriate than previous default UFs to derive tolerable daily intake values.

Is age an independent risk factor for chemically induced acute myelogenous leukemia in children?

Secondary or therapy-related acute myelogenous leukemia (t-AML) is a rare but unfortunate consequence of treatment with certain classes of cytotoxic chemotherapeutic agents or chronic exposure to high concentrations of benzene. Drugs known to produce AML following chemotherapy of primary malignancy are usually alkylating agents or topoisomerase II inhibitors. Both children and adults develop AML following treatment with these classes of antineoplastic drugs. In this review, the effect of age at treatment on a child's susceptibility to developing therapy related AML was investigated. The clinical literature describing pediatric cancer patients treated with cytotoxic chemotherapeutic agents was used to characterize risk factors associated with chemical leukemogenesis in children. As demonstrated in the published literature, the risk of developing AML following chemotherapy is not reliably correlated with the age of the pediatric patient. There is no consistent evidence that indicates that younger children will be at increased risk; in fact, some studies suggest that younger children might actually display a decreased susceptibility. The age dependency of treatment-related malignancies (all types) in children appears to vary considerably with the type of secondary neoplasm in question. For example, secondary solid tumors such as breast, central nervous system (CNS), bone, and thyroid cancer are highly dependent on the age of the patient at time of diagnosis and treatment; in contrast, an age dependency for t-AML risk was not observed in these same patient populations. Predictably, the induction of t-AML in children follows a rational dose-response relationship, with increasing doses of chemotherapy resulting in greater risk. Recent U.S. Environmental Protection Agency (EPA) cancer risk assessment guidance recommends a default assumption that children are inherently up to 10-fold more sensitive than adults to carcinogen exposures. Available scientific and medical literature does not support the hypothesis that children necessarily possess an increased risk of developing AML following leukemogenic chemical exposure.

Pediatric susceptibility to 18 industrial chemicals: A comparative analysis of newborn with young animals

We comprehensively re-analyzed the toxicity data for 18 industrial chemicals from repeated oral exposures in newborn and young rats, which were previously published. Two new toxicity endpoints specific to this comparative analysis were identified, the first, the presumed no observed adverse effect level (pNOAEL) was estimated based on results of both main and dose-finding studies, and the second, the presumed unequivocally toxic level (pUETL) was defined as a clear toxic dose giving similar severity in both newborn and young rats. Based on the analyses of both pNOAEL and pUETL ratios between the different ages, newborn rats demonstrated greater susceptibility (at most 8-fold) to nearly two thirds of these 18 chemicals (mostly phenolic substances), and less or nearly equal sensitivity to the other chemicals. Exceptionally one chemical only showed toxicity in newborn rats. In addition, Benchmark Dose Lower Bound (BMDL) estimates were calculated as an alternative endpoint. Most BMDLs were comparable to their corresponding pNOAELs and the overall correlation coefficient was 0.904. We discussed how our results can be incorporated into chemical risk assessment approaches to protect pediatric health from direct oral exposure to chemicals.

Differences between children and adults: implications for risk assessment at California EPA

The California legislature enacted a law requiring the California Environmental Protection Agency (Cal/EPA) Office of Environmental Health Hazard Assessment (OEHHA) to evaluate whether our risk assessment methodologies are adequately protective of infants and children. In addition both OEHHA and the California Air Resources Board must examine whether the Ambient Air Quality Standards set for criteria air pollutants and the health values developed for air toxics are adequately protective of infants and children. We have initiated a program to look at potential differences in response to toxicants between children and adults. We are evaluating this issue from the perspective of exposure differences as well as toxicokinetic and toxicodynamic differences between children and adults. Data on specific chemicals are rather limited. As a result, we will be pooling information to determine whether there are generic differences between children and adults that may be applicable to risk assessment in general or to risk assessment of specific classes of compounds. This paper discusses the rationale for approaching the issue of determining whether our risk assessment methods are adequate for infants and children and includes a discussion of some of the available information on both qualitative and quantitative differences in response to toxicants between children and adults or immature and mature laboratory animals. We provide examples of differences between children and adults in absorption, metabolism, and excretion of toxicants as well as qualitative differences in toxic response.

Differential sensitivity of children and adults to chemical toxicity. II. Risk and regulation

Animals can be useful predictors of chemical hazards to humans. Growth and development are compressed into a shorter period in animals, which makes interpretation of animal testing inherently more difficult. However, similar events occur in both humans and laboratory animals and testing that covers the full period of animal development can reasonably be considered an appropriate surrogate for human development. Some have proposed an additional 10-fold factor for the extra protection of children when estimating safe exposures. Use of such an additional factor, as required by the Food Quality Protection Act (FQPA), is meant to address the same issues covered by the EPA's database uncertainty factor, UF(D), and additional issues related to exposure uncertainty. Thus, when UF(D) has already been deployed, the EPA modifies its use of the FQPA factor. Based on our analysis, we agree with the EPA. Drawing conclusions about the adequacy of UF(H), the uncertainty factor used to account for intrahuman variability, in terms of its ability to protect children on the basis of the modest data available is challenging. However, virtually all studies available suggest that a high percentage of the population, including children, is protected by using a 10-fold uncertainty factor for human variability or by using a 3.16-fold factor each for toxicokinetic and toxicodynamic variability. Based on specific comparisons for newborns, infants, children, adults, and those with severe disease, the population protected is between 60 and 100%, with the studies in larger populations that include sensitive individuals suggesting that the value is closer to 100%.

Clinical research in paediatric oncology and the role of the research nurse in the UK

Survival rates for childhood cancer have improved considerably as new drugs, treatment protocols and supportive therapies have developed through clinical trials. Such studies take a considerable amount of time and organization and alongside scientific and medical staff the research nurse plays a vital role. The facets of this role will be outlined in this paper. Involvement begins at an early stage of study development and includes applying for Research Ethics Committee approval of the project. Once the study has opened the research nurse is then responsible for patient recruitment, monitoring and follow-up. The research nurse works within a team but also with a degree of autonomy ensuring that standards of patient care are maintained by adhering to guidelines for clinical research in general and those aimed at children specifically. Providing detailed information and support to the child and family, staff and outside agencies are other notable aspects of the post. The role of the research nurse continues to develop, as clinical trials maintain a significant role in improving the treatment for childhood cancer.

Differences in sensitivity of children and adults to chemical toxicity: the NAS panel report

The National Academy of Sciences (NAS) Committee on Pesticides in the Diets of Infants and Children worked for some 4 years to evaluate the extent and the health-related consequences of exposure of infants and children to pesticides. The focus of this paper is on deliberations and recommendations of the committee relevant to protection of infants and children from toxic effects of pesticides. The most comprehensive data available for contrasting the toxicity of chemicals in the young and adults were compilations of rodent mortality studies. Age-dependent differences in chemical lethality were less than 1 order of magnitude and usually varied no more than 2- to 3-fold. Findings in studies of pesticides and other chemicals revealed that toxicity was age- and compound-dependent. The younger and more immature the subject, the more different its response from that of an adult. Substantial anatomical, biochemical, and physiological changes occur during infancy, childhood, and adolescence. These maturational changes can substantially affect the absorption, distribution, metabolism, and elimination of chemicals. The net effect of immaturity on pharmacokinetics and pharmacodynamics is difficult to predict. Measurements of physiological functions in different age groups can be made and input into physiologically based pharmacokinetic (PBPK) models. The committee felt that PBPK models could be effectively utilized for different exposure scenarios, to predict the time course of potentially toxic chemicals and metabolites in different organs of children. The committee recognized that maturing organ systems of infants and children may be susceptible to injury by chemicals. There may be developmental periods (i.e., windows of vulnerability) when the endocrine, reproductive, immune, visual, or nervous systems are particularly sensitive to certain chemicals. The committee recommended early assessments using sensitive indices of injury to these organ systems of test animals. Only limited information was available on the therapeutic efficacy and toxicity of drugs in pediatric populations. The most definitive data were maximally tolerated doses (MTDs) of chemotherapeutic agents. MTDs were frequently higher for children than adults, though the differences between age groups were usually < or =2. It was concluded by the NAS committee that immaturity does not necessarily entail greater sensitivity to chemical toxicity; age-dependent toxicity is chemical-dependent; and the existing 10-fold interspecies uncertainty factor provides adequate protection of infants and children, based on current knowledge.

Pediatric phase II cancer chemotherapy trials: a Pediatric Oncology Group study

PURPOSE

This study reviewed the Pediatric Oncology Group experience with phase II clinical trials in children (< 21 years of age) with refractory tumors.

PATIENTS AND METHODS

Patients registered in Pediatric Oncology Group phase II studies were evaluated. Patients had to be < 21 years of age with recurrent and refractory measurable disease. Tumor types and response rates were determined. Death on therapy from either drug toxicity, progressive disease, infection, or hemorrhage was measured. Tumor-specific, disease-free survival curves were calculated by Kaplan-Meier analysis.

RESULTS

Between 1984 and 1994, 2,465 patient entries were made on 45 phase II trials. Malignancies registered included acute lymphocytic leukemia (ALL) (16.7%), acute myeloid leukemia (AML) (12.0%), osteogenic sarcoma (7.8%), neuroblastoma (7.2%), astrocytoma (7.2%), medulloblastoma (7.1%), glioma (6.7%), ependymoma (6.1%), and others (29.2%). The overall response rate was 19.6% (CR + PR) for children entered on phase II trials. Tumor-specific response rates ranged from 62.1% (23/37) for children with Hodgkin's disease to no responses (0/23) in patients with hepatoblastoma. When comparing single versus multiagent trials, a significantly better initial response rate was seen in the latter studies. However, 5-year survival was comparable. Progression-free survival for all tumor histologies were 12.9% and 9.2% at 2 and 5 years, respectively. Death on study was seen in 11.6% of the patients; however, only three deaths were directly related to drug toxicity. There were no significant gender differences in regards to response, progressive disease, or death on study.

CONCLUSION

Phase II studies conducted in children offer a considerable likelihood of therapeutic benefit without exposing these patients to untoward toxicity.

Adverse drug reactions from birth to early childhood

Neonates and older infants are a diverse group of children, quite different from their older counterparts. Adverse drug reactions may have profound immediate, delayed, and long-term implications for their neurologic and somatic development. The intrauterine, neonatal, and infancy periods are the only stages in life in which one is exposed to and affected by drugs administered to another person, the mother. In addition, because of the fragility of the neonate and the complexity of their illnesses, their pharmacotherapy is frequently complicated with misadventure and adverse drug reactions that are unavoidable or difficult to assess. Because of their differences in morphology and disease process and treatments, infants and children experience a different range of adverse drug reactions. These reactions are not necessarily predictable from the adult experience. Despite the advances made in the field of pediatric adverse drug reactions and the lessons learned through the misfortunes involving children, children continue to suffer. Sixty years after the Elixir of Sulfanilamide-Massengill disaster, children continue to be given medications with diethylene glycol in developing countries. Pediatricians, pharmacologists, and others must continue to be vigilant and active in preventing, monitoring, and treating adverse drug reactions in children. Learning from mistakes of the past will improve the health of children by preventing mistakes in the future.

Pediatric phase I drug tolerance: a review and comparison of recent adult and pediatric phase I trials

PURPOSE

We evaluated the ratio of pediatric to adult maximum tolerated doses (MTDs) from 70 Phase I studies conducted between 1975 and 1995. The aim of this study was to determine whether previously observed differences in drug tolerance between adult and pediatric Phase I patients have persisted over the 20-year period of this analysis.

PATIENTS AND METHODS

Phase I trials of pediatric and adult patients with solid tumors as the predominant diagnosis and sharing similar dosing regimens were evaluated. For consistent comparison between Phase I studies, the MTD was defined as the drug dose one level below that yielding dose-limiting toxicity in >30% of patients. The ratio of pediatric to adult MTDs was calculated and plotted chronologically by year of pediatric study closure. Statistical evaluation of MTD ratios included regression and correlation analysis. The extent of therapy before Phase I study entry was also examined.

RESULTS

Ninety-three Phase I studies were reviewed. Twenty-one drugs (70 studies) met our criteria for paired review of MTDs and analysis of the variation of ratio with time. The pediatric to adult MTD ratios ranged from 0.4 to 2.8, with a median of 1.2. Regression analysis of the ratio of MTD versus date of pediatric study closure supports a linear relationship of decreasing ratio with time (p<0.01). Analysis of the regression line predicts MTD ratios of 2.02 and 0.76 for 1974 and 1995, respectively. Of patients included in this analysis, 37.1% and 68.6% of adult and pediatric patients, respectively, were considered to have been heavily pretreated before study entry. A significant (p<0.001) downward trend with time was observed in the proportion of adult patients entering Phase I studies who had received both radiation and chemotherapy.

CONCLUSIONS

The results of this review continue to show an equal or greater drug tolerance in the pediatric population when compared with adult patients for most drugs studied during Phase I trials. However, there appears to be significant trend of decreasing differences in drug tolerance between pediatric and adult Phase I patients with time, as defined by the descent of the MTD ratio toward values <1.0. Mechanisms to explain greater drug tolerance in children and the observation of decreasing maximum tolerated dose ratios with time are discussed. Limited data suggest that changes in degree of therapy before Phase I study entry may be influencing the MTD ratio.

Pediatric Clinical Trials

Clinical trials specifically tailored to the unique tumors and leukemias of children have resulted in increased survival rates approaching or exceeding 70% for most diseases. These studies have been carried out by investigators at large, independent institutions or through the auspices of the Children's Cancer Group or Pediatric Oncology Group. The National Cancer Institute has also supported pediatric disease—focused efforts for rhabdomyosarcoma, Wilms tumor, Ewing's sarcoma, osteosarcomas and other diseases. The present-day training of pediatric hematology/oncology fellows assures continuing contributions to the biology of childhood malignant lesions through applications of translational research.

The conduct of phase I-II clinical trials in children with cancer

Phase I Clinical Trials in children with cancer are carried out with therapeutic intent, so as to define antileukemic/antitumor activity, dose-limiting toxicity, maximum tolerated dosage, and pharmacokinetics. These studies define a dosage of an agent or combination that may be used in a larger study of pediatric patients with leukemias or malignant solid tumors. The content of phase I and II protocols is outlined, with a brief discussion of the ethics of performance of phase I trials in children.

Treatment results among adults with childhood tumors: a 20-year experience

Controversy exists regarding the most appropriate treatment for the rare adult patient who develops a so-called pediatric cancer. We have reviewed our 20-year experience with these patients and analyzed their outcome. A total of 299 patients with rhabdomyosarcoma (106), Wilms' tumor (97), and neuroblastoma (96) were evaluated and treated at Stanford University Medical Center between January 1967 and December 1987. Only 26 of these patients (8.7%) were diagnosed during "adulthood"; their age range was 18-67 years, median 23 years. Wilms' tumor; Five patients presented with Wilms' tumor at age greater than or equal to 18 years; four had unfavorable histology. All underwent multimodality therapy; however, only two have survived, one currently disease-free and one with disease. Neuroblastoma: Five patients presented with neuroblastoma at age greater than or equal to 18 years. Four underwent attempted surgical resection, post-operative irradiation (RT), and chemotherapy (CT); the other received no adjuvant CT. Only two of the five patients survive, both with disease. Rhabdomyosarcoma: Of the 16 adults (greater than or equal to 21 years) with rhabdomyosarcoma, 14 (87%) had advanced Intergroup Rhabdomyosarcoma Study-group disease (eight Group III, six Group IV). All 16 underwent aggressive multimodality therapy. At 10 months-16 years follow-up, only five patients survive, four of whom are apparently cured of their tumor. Neither histologic subtype nor site of presentation were of prognostic value. This series demonstrates that adults with Wilms' tumor, neuroblastoma, or rhabdomyosarcoma have a worse prognosis than do children with the same diagnosis. Possible explanations for this disparity in outcome include different tumor biology, less tolerance for treatment, and different natural history among adults relative to children.

Neuro-ocular damage in pediatric oncology patients: predictor of long-term visual disability or tool for limiting toxicity?

We present a group of eight pediatric cancer patients with a spectrum of visual afferent pathway abnormalities. Changes include decreased visual acuity, visual field alterations, abnormal visual evoked potentials, changes in the optic disc and nerve fiber layer of the retina, radiation retinopathy, and CNS injury. These changes occur in long term survivors of pediatric malignancy (especially those with prolonged, multimodal, and multicourse therapy), but they may be minimally symptomatic. The changes appear to be analogous to the CNS changes (leukoencephalopathy) described in patients with leukemia and attributed to multimodal therapy. By taking advantage of opportunities to detect adverse effects earlier in the treatment course, the present excellent cure rate may be improved by refinements in therapy that also improve the quality of survival.

Cancer chemotherapy in infants and children

The narrow therapeutic index and potentially life-threatening toxicities of the antineoplastic agents require a clear understanding of their toxicologic and pharmacologic properties, particularly in infants and children. This article reviews the literature published in the last five years on pediatric cancer chemotherapy. It begins with an overview of new concepts in antineoplastic therapy and then summarizes each of the main drugs used to treat the cancer of infants and children.