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Background
The mission of the Developmental Therapeutics Program (DTP) of the Division of Cancer Treatment and Diagnosis (DCTD) of the National Cancer Institute (NCI) centers on the discovery and preclinical development of agents with clinical anti-cancer or anti-HIV potential. Investigations focusing on the hazards of these compounds to healthy organs in intact experimental animals are the final steps in the preclinical stages of new drug development. Data generated from these studies on each new drug are evaluated in light of potential human toxicity and form a major portion of the information required by the Food and Drug Administration (FDA) for an Investigational New Drug Application (IND). Such laboratory investigations comprise the primary responsibility of the Toxicology & Pharmacology Branch (T&PB) within the DTP.
Toxicology studies that are conducted by T&PB are designed to meet this responsibility include the following major objectives:
- Determination of a maximum tolerated dose (MTD).
- Determination of dose limiting toxicities (DLT).
- Determination of schedule-dependent toxicity.
- Determination of the reversibility of adverse effects.
- Determination of a safe clinical starting dose.
In addition to these objectives, these studies are also designed to make the following determinations:
- Whether the efficacious drug levels can be attained in plasma in vivo.
- Whether the drug crosses the blood-brain barrier (if required).
- Whether the drug is orally bioavailable (if required).
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Drug Evaluation Philosophy
Over the past ten years, the emphasis of preclinical toxicology studies has continued to shift away from the use of standardized protocols (such as Dx1 and Dx5 studies by intravenous administration) to evaluate the potential adverse effects of new anti-cancer agents. Current practice involves designing and performing agent-directed studies within a pharmacologically-guided framework. This is accomplished through the acquisition and use of pharmacokinetics information to reliably extrapolate toxic effects across species by relating plasma drug levels [peak and steady state and/or area-under-the-curve (AUC)] to safety and the occurrence and severity of toxicity. Integration of these studies with efficacy data and the proposed clinical protocol permits a more rational evaluation of the role of schedule dependence and pharmacokinetics in the development of toxicity.
In addition to this philosophical change in the approach to the development of toxicity information, the development of drugs within the DTP has evolved into two stages that require a toxicological evaluation at each stage. The first involves a preliminary assessment of toxicity in two preclinical animal models with the determination of maximum tolerated doses (MTD) in both species and pharmacokinetics in at least one the preclinical animal models. If the drug meets the program criteria for full-scale development, a more complete toxicity evaluation is performed that will lead to the filing of an IND. This two-step approach allows the program to determine if full development is reasonable with a limited expenditure of resources (drug, animals and funds).
Note: A listing of relevant publications that document the changes in cancer drug development over the last 30+ years is available.
Go to T&PB Drug Development Bibliography
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Drug Evaluation Procedures
Preclinical toxicology and pharmacology studies are conducted at the various DTP toxicology and pharmacology contractors in accordance with the complete protocol guidelines developed for each agent by the toxicologist or pharmacologist in the T&PB. It is inappropriate to set rigid guidelines that would be adequate for each drug, therefore, the general procedures below will be modified to account for data on the agent's in vitro cytotoxicity, biochemistry, pharmacology and schedule dependence as well as in vivo activity in tumored animal models, if available:
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Bioanalytical Method Development Phase: Develop and validate a bioanalytical method for quantitation of the compound in question in mouse, rat, dog and human plasma. Characterize the stability in plasma at 37 C. If significant decomposition is observed, similar studies should be conducted at 4 C or lower. Assess the stability in frozen plasma and/or plasma extracts to determine appropriate sample handling and storage procedures.
Determine the extent of protein binding in mouse, rat, dog and human plasma. Additional binding studies with constituent plasma proteins (albumen, 1-AGP) may also be performed.
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Pharmacokinetic Phase: Plasma distribution and elimination kinetics will be determined in one or more of the following species: rodents, dogs and non-human primates after single intravenous doses of drug. Of primary importance will be the determination of plasma levels or AUC associated with efficacy in tumored animal models. Other routes of administration such as oral, intraperitoneal, subcutaneous and intramuscular may be necessary to evaluate as well. Bioavailability of non-parenteral routes and plasma clearance rates will be determined in order to establish the dose required to produce effective drug concentrations in plasma for future toxicity study evaluation. The ability of a drug to cross the blood-brain barrier will be assessed in dogs or non-human primates, as needed.
A standard mathematical work-up of the plasma concentration-time data to yield half-life values, plasma clearance, AUC, etc. will be performed. Urine may be collected in selected experiments for determination of cumulative drug excretion, urinary clearance, and possible identification of drug metabolites. Major organs and tissues may be obtained at necropsy and frozen for possible characterization of the tissue distribution of the compound.
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Metabolism Phase: For those drugs that undergo metabolism, the in vitro and in vivo metabolism will be characterized and the potential effects of this compound on drug metabolizing enzymes will also be evaluated. Initially, the metabolism by mouse, rat, and human liver microsomes or S9 fractions may be characterized. These studies may include determinations of cofactor dependency and effects of CO and N2 on metabolism.
If P450 involvement is evident, studies using cDNA-expressed P450 isoforms and/or specific substrate/inhibitors of P450 isoforms shall be performed to further characterize the enzymes responsible for metabolism.
In vivo studies in mice and/or rats may be performed to confirm and extend the preliminary investigations described above and to obtain quantities of metabolites for structure identification and comparisons with in vitro results.
If results of the in vitro and in vivo studies outlined above suggest that the compound could induce its own metabolism and/or be involved in a clinically significant metabolism-based drug interactions, in vivo studies to test this hypothesis will be considered.
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Preliminary Toxicity Phase: For each drug, it will be necessary to establish a maximum tolerated dose (MTD) and dose limiting toxicities (DLT) in both rodent and non-rodent species. The following types of studies may be required in this phase:
- Up/Down toxicity study in rodents
- Single or multiple dose range-finding studies in rodents, beagle dogs, or non-human primates.
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IND-Directed Toxicity Phase: For each drug, it will be necessary to establish toxicity and safety in relation to drug plasma concentrations or area-under-the-curve in both rodent and non-rodent species. The following types of studies may be required in this phase:
- Single or multiple daily dose schedules such as Dx1, q3hr x 3, q8hr x 15, q4D x 3, etc.
- Continuous administration to mice via Alzet osmotic pumps and to beagle dogs, rats and non-human primates via infusion pumps for periods of from one hour up to 30 days.
- Up to twenty eight days or more of repeated administration of drug to rodents, beagle dogs and/or non-human primates.
- Multiple cycle studies may be required if delayed or cumulative toxicity is anticipated.
- Special studies such as cardiotoxicity, neurotoxicity evaluations, immunotoxicity studies may be required as part of an existing study or in a separate study.
- Human, canine and murine bone marrow toxicity assessment in vitro and correlation with toxicity occurring in vivo.
Toxicity Data Tables of Selected Cancer Drugs
In Vitro Bone Marrow Toxicity Data of Selected Cancer Drugs
Correlation of Clinical and Preclinical Cancer Drug MTDs
Correlation of Clinical and Preclinical Cancer Drug DLTs
Note: Representative protocols of the various toxicity studies are available.
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