Evaluation of the efficiency of targeting of antitumor drugs: simulation analysis based on pharmacokinetic/pharmacodynamic considerations

In vivo antitumour efficacy of MGI-114 (6-hydroxymethylacylfulvene, HMAF) in various human tumour xenograft models including several lung and gastric tumours

MGI-114 (6-hydroxymethylacylfulvene, HMAF) is a semi-synthetic analogue of the cytotoxic sesquiterpenoid illudins. In the present study, the in vivo antitumour efficacy of MGI-114 was examined in a panel of human tumour xenograft models consisting mainly of human lung and gastric tumours, and compared with that of other antitumour drugs such as irinotecan, paclitaxel, cisplatin, doxorubicin, vindesine, etoposide and 5-fluorouracil (5-FU). When different administration schedules were compared, daily administration of MGI-114 was found to be more effective than intermittent administrations. In human tumour xenograft models of nasopharyngeal, breast and colon carcinoma and melanoma, MGI-114 exerted a strong antitumour activity with complete tumour regression being observed. Moreover, in four human lung and three gastric tumour xenograft models, MGI-114 showed a strong antitumour activity with complete tumour regression being observed in some of the models. The antitumour efficacy of MGI-114 was generally higher than or equivalent to that of other antitumour drugs such as irinotecan and paclitaxel. These results support the potential utility of MGI-114 in the treatment of a variety of human solid tumours.

Determinants for the drug release from T-0128, camptothecin analogue-carboxymethyl dextran conjugate

To improve pharmacological profiles of camptothecins (CPTs), a new macromolecular prodrug, denoted T-0128, was synthesized. This prodrug comprises a novel CPT analog (T-2513: 7-ethyl-10-aminopropyloxy-CPT) bound to carboxymethyl (CM) dextran through a Gly-Gly-Gly linker, with a molecular weight of 130 kDa. The present study was designed to elucidate the mechanisms that promote the release of linked T-2513. First, we compared the abilities of a rat liver homogenate, a cocktail of its lysosomal enzymes, and different types of pure enzymes, to liberate T-2513 from the conjugate. The releasing rate in the homogenate was very slow, but was accelerated with the lysosomes. Lysosomal cysteine proteinases, such as cathepsin B, were responsible, coupled with the results of in vitro and in vivo inhibition studies using proteinase inhibitors. The pH optimum for the cathepsin B-mediated drug release was approximately 4. This corresponds to the pH in lysosomes, suggesting lysosomotropic release. Second, to assess the effect of the length and composition of the peptidyl linker, we synthesized the conjugates with a different linker and compared the drug-releasing rates. We found that the insertion of Phe into Gly-Gly-Gly allowed various kinds of enzymes to produce a rapid cleavage, and the Gly-chain lengthening enhanced the lysosome-mediated drug release. The released T-2513 levels in the liver and tumor of the tumor-bearing rats dosed with each conjugate increased with the length of Gly linker, suggesting a good in vitro to in vivo relationship. Comparative efficacy studies of the conjugates with a different linker demonstrated that T-0128 showed the maximum efficacy against MX-1 human mammary xenograft tumors. Thus the Gly-Gly-Gly linker exploits lysosomal cathepsin B to liberate T-2513 slowly and steadily, resulting in improved therapeutic efficacy.

Pharmacokinetic/pharmacodynamic modeling of antitumor agents encapsulated into liposomes

Pharmacokinetic/pharmacodynamic (PK/PD) modeling of antitumor agents has been developed for doxorubicin (DOX) in order to predict the optimum conditions for a drug carrier to maximize the antitumor effect. A PK model was constructed for free and liposomal doxorubicin using a hybrid model wherein the disposition in the whole body is described by compartment models, which were linked to the tumor compartment via the blood flow rate. The PD model for doxorubicin was described by a cell-kill kinetic model, which represents the number of tumor cells quantitatively, as a function of the free concentration of doxorubicin in the tumor compartment. The influence of each parameter on the antitumor effects was examined by sensitivity analysis based on the PK/PD model, which clearly showed the importance of optimizing the release rate of DOX from liposomes. The validity of the model has been tested using animal experiments. Preliminary simulations were also performed for humans after scaling up the PK/PD model from rodents to humans. The optimum conditions in the rate of drug release from liposomes were different for rodents vis-a-vis humans, which indicates the limitations involved in extrapolating optimum conditions for experimental animals to those for humans.

Optimization of antitumor effect of liposomally encapsulated doxorubicin based on simulations by pharmacokinetic/pharmacodynamic modeling

It has been reported that long circulating liposomes enhanced the antitumor effect of doxorubicin (DOX) by increasing delivery of DOX to tumor tissues. However, there is no quantitative information on the relationship between the antitumor effect and liposomal characteristics governing the release rate of entrapped drugs, although the importance of drug release-rate control from liposomes has been pointed out. Here, we developed a physiological model for free and liposomal DOX to calculate the time course of free DOX in the extracellular space and linked this with a cell kill kinetic model to quantify the antitumor effect of DOX. Simulations were performed to clarify the relationship between antitumor effect and pharmacokinetic or physicochemical parameters of liposomes, as well as pharmacological or physiological parameters of tumor tissues. The importance of long circulation time of liposomes was confirmed. The optimum rate of drug release from long circulating liposomes was found at the release rate constant of around 0.06 h(-1). This optimum value was not dependent on the tumor proliferation time, sensitivity of tumor cells to DOX, or the tumor blood flow-rate. This simulation indicated that the optimization of the delivery to tumor tissue by long circulating liposomes could be possible by changing the release rate of DOX for the maximum antitumor effect.

Pharmacodynamic aspects of sustained release preparations

The sustained release (SR) mode of drug administration has certain features that have an important impact on the magnitude of the pharmacologic response: (a) it minimizes fluctuation in blood drug concentrations (i.e. between peak and trough). However, due to the pronounced non-linear relationship between drug concentration and pharmacologic effect (i.e. pharmacodynamics) the impact of this property differs considerably as a function of the shape of the pharmacodynamic profile and the position of the specific range of concentrations on the curve of this profile; (b) it produces a slow input rate which tends to minimize the body's counteraction to the drug's intervening effect on regulated physiological processes; and (c) it provides a continuous mode of drug administration. This important pharmacodynamic characteristic may produce, in certain cases, an opposite clinical effect than that attained by an intermittent (pulsatile) mode of administration of the same drug. For many drugs with non-concentration-dependent pharmacodynamics, the exposure time, rather than the AUC, is the relevant parameter and it can therefore be optimized by SR preparations. The slow input function may minimize hysteresis in cases where the site of action is not in a rapid equilibrium with the blood circulation. The pharmacodynamics of the desired effect(s) and/or adverse effect(s) may also be influenced by the site of administration, especially in cases where the drug is delivered directly to its site of action. These factors demonstrate the important influence of the mode of administration on the pharmacological and clinical outcomes. In addition, they highlight the need to include these pharmacodynamic considerations in all stages from drug development to the optimization of their clinical use.