Macromolecular drug carrier systems in cancer chemotherapy: macromolecular prodrugs

Synthesis and in vitro evaluation of potential anti-leishmanial targeted drugs of pyrimethamine

Pyrimethamine, an antimalarial drug, was found to be able to inhibit both enzymes (DHFR-TS and PTR1) of the leishmanial folate pathway, although this effect in vivo appears only in relatively high concentrations. To reach the parasites inside macrophage cells, where they are sheltered, targeted drugs of pyrimethamine, carboxymethyldextran-thiomannopyranoside-pyrimethamine (CMD-P), and succinyldextran-thiomannopyranoside-pyrimethamine (SD-P), were synthesized and assayed against L.(L.) amazonensis amastigotes. CMD-P has 2.43% and SD-P has 2.58% of pyrimethamine attached. At a CMD-P dose of 200 microg/mL (4.86 microg/mL pyrimethamine), the results were very promising, with a destruction of approximately 50% of the intracellular amastigotes, with no detectable toxicity to macrophage cells. SD-P in similar doses did not show good results, probably due to different patterns of drug release. These results open the possibility of treating leishmaniasis with a safe targeted drug of pyrimethamine released directly inside the macrophage cells, reducing the host systemic toxicity.

Novel daunorubicin-carrier peptide conjugates derived from human calcitonin segments

Severe and often therapy-limiting side effects are a major obstacle in cancer chemotherapy. New delivery concepts reducing systemic side effects are needed in order to optimize anticancer therapies. Several approaches have been followed, most of them concentrating on macromolecular carriers like liposomes, monoclonal antibodies, serum proteins or polyethylene glycol. We present here a novel type of anthracycline conjugate, using a small carrier peptide derived from the peptide hormone human calcitonin (hCT). The carrier peptide hCT(9-32) has so far been shown to be capable of transporting fluorophores or proteins across cellular membranes. Two different carrier peptide-daunorubicin conjugates were prepared, one with an acid-stable amide bond, the second with an acid-labile hydrazone bond. In vitro studies with daunorubicin linked to the carrier peptide via an acid-labile hydrazone bond demonstrated comparable cytotoxicity to daunorubicin in various daunorubicin sensitive cell lines (neuroblastoma cell lines SK-N-MC and SMS-KAN; HEK 293 T cells). In addition, fluorescence microscopy provided further insight into the mechanism of uptake of the carrier peptide hCT(9-32), indicating that endosomal compartments with reduced pH are involved in the intracellular release of daunorubicin.

Tumor targeting based on the effect of enhanced permeability and retention (EPR) and the mechanism of receptor-mediated endocytosis (RME)

This review is focused on the macromolecular drug carrier systems by the effect of enhanced permeability and retention (EPR) and the mechanism of receptor-mediated endocytosis (RME). The effect of EPR is thought to be useful for the targeting of the macromolecular drugs to the tumor tissues on a vasculolymphatic level. The RME reveals the selective recognition, high affinity binding, and immediate internalization for the ligand on a cellular level. In the receptor, recognizing transferrin, a level of expression on the tumor cells is higher than that on the normal cells. We have used serum albumin and transferrin as drug carriers to deliver mitomycin C (MMC) to the tumor tissues and into the tumor cells. The properties of the conjugates of MMC to serum albumin and transferrin were examined in vitro and in vivo. We concluded that MMC could be delivered to the tumor tissue and cells by the use of albumin and transferrin as drug carriers.

Physicochemical characterization of branched chain polymeric polypeptide carriers based on a poly-lysine backbone

A systematic study is reported on the physicochemical characteristics of two branched chain polymers (based on a poly-L-lysine backbone) with a general formula poly[Lys-(DL-Alam-Xi)], where X = Orn (OAK) or N-acetyl-Glu (Ac-EAK) and m approximately equal to 3, using surface pressure and fluorescence polarization methods. These data are compared with those of the linear poly(L-Lys) from which OAK and Ac-EAK are derived. These two polymers show a moderate surface activity, able to form stable monomolecular layers at the air-water interface. Poly(L-Lys), the most hydrophilic, has the lowest surface activity. The interaction of these polymers with phospholipid bilayers either neutral or negatively charged was studied with vesicles labeled with two fluorescent probes: ANS and DPH. Results indicate that these polymers are able to accommodate in their internal structure, mainly through electrostatic interactions, a certain amount of ANS marker molecules, but fluorescence increases of the ANS-polypeptide complexes were so low that its influence in further polarization measurements could be discarded. After interaction with liposomes, these polymers induce an increase in the polarization of the probes, thus indicating a rigidification of the bilayers. Electrostatic forces seem to be very important in this interaction; cationic polymers are clearly more active, with PG-containing liposomes, than Ac-EAK. Moreover, in these assays poly(L-Lys) behaves as the more active compound. This fact is probably due to its major ability to form alpha-helical structures that could insert easily in the bilayers. These results indicate that the polymeric structures studied can be used as carriers for biologically active molecules, because their interactions with bilayers remain soft and have a positive effect on the stability of the membranes.

Drug targeting by macromolecules without recognition unit?

his review will summarize available information on the ability of macromolecular conjugates containing no specific recognition motifs to deliver anthracyclines (daunomycin, adriamycin) or methotrexate to target cells such as tumour cells or macrophages. Conjugates with natural (proteins, DNA, carbohydrates) and synthetic macromolecules (linear and branched chain poly-alpha-amino acids, non-biodegradable DIVEMA, HPMA etc.) will be reviewed. Experimental data from several laboratories indicate that these conjugates are taken up by cells mainly by fluid-phase or adsorptive endocytosis. It is believed that these processes do not involve 'specific receptors'. Two examples of methotrexate and daunomycin conjugates will be discussed to show the effect of the chemical structure of branched chain polypeptides on the uptake and antitumour or antiparasitic (Leishmania donovani infection) efficacy of conjugates.

Isomer-dependent daunomycin release and in vitro antitumour effect of cis-aconityl-daunomycin

Two isomers of cis-aconytil-daunomycin (cAD) were isolated after the reaction of daunomycin with cis-aconitic-anhydride. The structure of the isomers was identified by MS-spectroscopy and 1H and 13C NMR experiments. In contrast with the assumptions described earlier, our results show that the two isomers belong to the cis- and trans-isomers of the alpha-monoamide of cis-aconityl-daunomycin, respectively. We found that the pH dependent daunomycin release is different for the two isomers. Comparative analysis of the in vitro antitumour effect of the isomers on c26 colon carcinoma and on MDA-MB 435P human breast carcinoma cell lines showed that cAD-1 is more potent than cAD-2, but the extent of differences is tumour cell dependent. The results of this study might be appreciated in the light of the use of acid-labile spacer for the design and preparation of protein/peptide conjugates of drugs by indicating that isomers could possess markedly different biological activity.

Paclitaxel delivery systems: the use of amino acid linkers in the conjugation of paclitaxel with carboxymethyldextran to create prodrugs

Paclitaxel was bound via its hydroxyl group to carboxymethyldextran (CMDex, 150 kDa) by means of an amino acid linker; the linker was introduced into the 2'- or 7-hydroxyl group of the paclitaxel through an ester bond. These conjugates--CMDex-2'-paclitaxel and CMDex-7-paclitaxel--were designed to be water-soluble with a paclitaxel content between 6-8% (w/w) with a degree of subsititution (DS) of the CM groups at 0.6 per sugar residue. The release of the paclitaxel from the conjugates was influenced by the hydroxyl group (2'- or 7-) of paclitaxel to which the amino acid linker was introduced, and by what amino acid was used as the linker. In mouse plasma incubated at 37 degrees C for 72 h, the most paclitaxel was released using CMDex-paclitaxel conjugate with 2'gly followed by, in descending order, 2'-ala, 2'-leu, 2'-ile, and 7-gly as the amino linkers. Colon 26, a Taxol resistant cancer, was introduced into mice and the conjugates were intravenously administered by bolus injection for a tumor distribution study, and intermittently intravenously administered for a tumor growth regression study. In both studies the highest amount of paclitaxel release was found in the CMDex-2'-gly-paclitaxel followed by CMDex-2'-ala-paclitaxel, CMDex-2'-leu-paclitaxel and paclitaxel. There was a direct correlation between the amount of paclitaxel released and the observed efficacy. CMDex-2'-ile-paclitaxel and CMDex-7-gly-paclitaxel did not show any anti-tumor activity. These results clearly demonstrate that a CMDex-paclitaxel with an appropriate amino acid linker has significant anti-tumor activity against colon 26, and that these anti-tumor effects appear to correlate with the amounts of paclitaxel released in the tumor.

Dextran-methylprednisolone succinate as a prodrug of methylprednisolone: dose-dependent pharmacokinetics in rats

The dose-dependency in the pharmacokinetics of a macromolecular prodrug of methylprednisolone (MP), dextran-methylprednisolone succinate (DMP), was investigated in rats. Single doses (MP equivalent) of 2.5, 5.0, 10, 20, and 30 mg/kg of DMP were administered intravenously to rats (n=5/group), and serial blood samples (0-96 h) and spleen and liver tissues (96 h) were collected. The concentrations of DMP in plasma and spleen were analyzed using a size-exclusion chromatographic method. The concentrations of DMP in the liver samples were determined by an indirect method after sequential hydrolysis by dextranase and esterase enzymes, followed by HPLC analysis of MP. The kinetics of DMP were analyzed by non-compartmental methods. The systemic clearance of DMP decreased approximately 5-fold (from 42.1+/-11.0 to 7.72+/-1.84 ml/h per kg) when the dose was increased from 2.5 to 30 mg/kg. The nonlinearity in the clearance of DMP could be adequately described by a Michaelis-Menten type elimination with a maximum velocity of elimination of 1.72 mg/h per kg and a constant of 24.9 microg/ml. Additionally, the percent of the dose of DMP found at 96 h in the liver and spleen, where the prodrug is sequestered and gradually eliminated, significantly decreased with an increase in the dose. It is concluded that the clearance of DMP in rats is modestly dose-dependent in the dosage range of 2.5-30 mg/kg.

In vitro and in vivo efficacy of acid-sensitive transferrin and albumin doxorubicin conjugates in a human xenograft panel and in the MDA-MB-435 mamma carcinoma model

Acid-sensitive transferrin and albumin conjugates with doxorubicin have recently been developed with the aim of circumventing the systemic toxicity and improving the therapeutic efficacy of this anticancer agent. The in vitro activity of two acid-sensitive transferrin and albumin doxorubicin conjugates and free doxorubicin was evaluated in twelve human tumour xenografts using a clonogenic assay. The inhibitory effects and the activity profile of the conjugates was, in general, comparable to that of doxorubicin (mean IC(70) -value for doxorubicin approximately 0.1 microM and 0.1 - 0.4 microM for the conjugates). Subsequently, the efficacy of an acid-sensitive transferrin and albumin doxorubicin conjugate, which both incorporated a phenylacetyl hydrazone bond as a predetermined breaking point, was evaluated in the xenograft mamma carcinoma model MDA-MB-435 in comparison to free doxorubicin (dose, i.v.: 2 x 4, 8 and 12 mg/kg). The conjugates showed significantly reduced toxicity (reduced lethality and body weight loss) with a concomitantly stable or slightly improved antitumour activity compared to free doxorubicin. At the dose of 12 mg/kg mortality was unacceptably high in the doxorubicin treated group ( approximately 80%); in contrast, no mortality was observed with the conjugate treated animals with body weight loss < 10 %. In a further experiment, therapy with the acid-sensitive doxorubicin albumin conjugate at 3 x 12 mg/kg in the MDA-MB-435 model resulted in a significantly improved antitumour activity over free doxorubicin at its optimal dose of 2 x 8 mg/kg. In conclusion, acid-sensitive transferrin and albumin doxorubicin conjugates can be administered at higher doses than free doxorubicin in nude mice models with a concomitant improvement in antitumour activity. Interestingly, there is no pronounced difference between identically constructed transferrin and albumin doxorubicin conjugates with regard to in vitro or in vivo efficacy.

Novel peptide conjugates for tumor-specific chemotherapy

One of the major problems in cancer chemotherapy are the severe side effects that limit the dose of the anticancer drugs because of their unselectivity for tumor versus normal cells. In the present work, we show that coupling of anthracyclines to peptides is a promising approach to obtain selectivity. The peptide-drug conjugate was designed to bind to specific receptors expressed on the tumor cells with subsequent internalization of the ligand-receptor complex. Neuropeptide Y (NPY), a 36-amino acid peptide of the pancreatic polypeptide family, was chosen as model peptide because NPY receptors are overexpressed in a number of neuroblastoma tumors and the thereof derived cell lines. Daunorubicin and doxorubicin, two widely used antineoplastic agents in tumor therapy, were covalently linked to NPY via two spacers that differ in stability: an acid-sensitive hydrazone bond at the 13-keto position of daunorubicin and a stable amide bond at the 3'-amino position of daunorubicin and doxorubicin. Receptor binding of these three conjugates ([C(15)]-NPY-Dauno-HYD, [C(15)]-NPY-Dauno-MBS, and [C(15)]-NPY-Doxo-MBS) was determined at the human neuroblastoma cell line SK-N-MC, which selectively expresses the NPY Y(1) receptor subtype, and cytotoxic activity was evaluated using a XTT-based colorimetric cellular cytotoxicity assay. The different conjugates were able to bind to the receptor with affinities ranging from 25 to 51 nM, but only the compound containing the acid-sensitive bond ([C(15)]-NPY-Dauno-HYD) showed cytotoxic activity comparable to the free daunorubicin. This cytotoxicity is Y(1) receptor-mediated as shown in blocking studies with BIBP 3226, because tumor cells that do not express NPY receptors were sensitive to free daunorubicin, but not to the peptide-drug conjugate. The intracellular distribution was investigated by confocal laser scanning microscopy. We found evidence that the active conjugate [C(15)]-NPY-Dauno-HYD releases daunorubicin, which is localized close to the nucleus, whereas the inactive conjugate [C(15)]-NPY-Dauno-MBS is distributed distantly from the nucleus and does not seem to release the drug within the cell.

Polymeric prodrug for release of an antitumoral agent by specific enzymes

The clinical usefulness of antitumor chemotherapy has been strongly limited by the lack of specificity of most anticancer drugs, which act also against healthy cells. The aim of this work was to design, synthesize, and evaluate a macromolecular prodrug of Cytarabine, a known antitumor drug, which is a specific substrate for plasmin enzyme whose concentration is high in various kinds of tumor mass as a result of plasminogen activator secretion. alpha,beta-Poly(N-hydroxyethyl)-DL-aspartamide (PHEA), a known synthetic and biocompatible polyamino acid, was used as a drug carrier, and Cytarabine was linked to PHEA by D-Val-Leu-Lys spacer synthesized beginning from Cbz-D-Val-LeuOH dipeptide and N6-CbzLys methyl ester. The content of Cytarabine in the purified PHEA-D-Val-Leu-Lys-Cytarabine conjugate was equal to 3% w/w. In vitro experiments in the presence of plasmin evidenced the ability of this enzyme to strongly increase drug release from the macromolecular prodrug, as well as plasma incubation shows high stability of drug-polymer linkage. The direct linkage of Cytarabine to PHEA was also performed and, like PHEA-D-Val-Leu-Lys-Cytarabine conjugate, the obtained PHEA-Cytarabine conjugate showed high stability in plasma, but no release of Cytarabine was revealed in the presence of plasmin.

Differences in the intracellular distribution of acid-sensitive doxorubicin-protein conjugates in comparison to free and liposomal formulated doxorubicin as shown by confocal microscopy

PURPOSE

To investigate differences in the cellular uptake and intracellular distribution of protein-bound doxorubicin in comparison to free doxorubicin and a liposomal formulation (CAELYX) METHODS: LXFL 529 lung carcinoma cells were incubated with an acid-sensitive transferrin and albumin conjugate of doxorubicin, a stable albumin doxorubicin conjugate, and free and liposomal doxorubicin for up to 24 h. The uptake of doxorubicin was detected with confocal laser scanning microscopy (CLSM). To investigate the intracellular localization of the anticancer drug, lysosomes, Golgi apparatus, and mitochondria were also stained by various organelle-specific fluorescent markers. In vitro efficacy of the doxorubicin derivatives was examined with the BrdU incorporation assay.

RESULTS

The acid-sensitive albumin and transferrin doxorubicin conjugates showed enhanced cytotoxicity in comparison to liposomal doxorubicin, whereas the stable albumin-doxorubicin conjugate showed only marginal activity. Of all compounds tested, doxorubicin showed the highest cytotoxicity. CLSM studies with specific markers for lysosomes, mitochondria, and the Golgi apparatus demonstrated that protein-bound doxorubicin or liberated doxorubicin was accumulated in the mitochondria and Golgi compartments, but not in the lysosomes after 24 h. Free doxorubicin showed a time-dependent intracellular shift from the nucleus to the mitochondria and Golgi apparatus. Fluorescence resulting from incubation with CAELYX was primarily detected in the nucleus.

CONCLUSIONS

Our results indicate that other organelles in addition to the cell nucleus are important sites of accumulation and interaction for protein-bound doxorubicin or intracellularly released doxorubicin as well as for free doxorubicin.

Disposition characteristics of emulsions and incorporated drugs after systemic or local injection

Lipid emulsions are useful tools for controlling the in vivo disposition of drugs and plasmid DNA. The dispositions of lipid emulsions are determined by their tissue interaction depending on the anatomical and physiological characteristics of each tissue and the physicochemical and biological properties of lipid emulsions. In addition, the retention of drugs is another issue, as too rapid a release of the drug would lead to failure of exerting its therapeutic potency. This review presents an overview about the disposition profiles and various physicochemical properties of lipid emulsions and incorporated drugs after systemic or local injection. Controlled biodistribution of lipid emulsions and incorporated drugs are also discussed.

Characterization and in vitro methotrexate release from methotrexate/gelatin conjugates of opposite conjugate bond polarity

PURPOSE

Our laboratory has previously prepared gelatin/ methotrexate (MTX) conjugates containing mixed conjugation sites and by-product crosslinking, both of which may alter conjugate effectiveness. In this study, we prepared and evaluated gelatin/MTX conjugates having specific conjugate bond sites and minimal by-product crosslinking.

METHODS

Opposite polarity conjugates were produced by coupling gelatin having blocked amino groups with MTX (G-MTX) and by coupling MTX having blocked amino groups with gelatin (M-GEL) using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide HCl. Amino groups were blocked using citraconic anhydride and deblocked under acidic conditions. Gelatin and MTX contents were determined spectrophotometrically. The stability of each conjugate was determined by evaluating their in vitro release of MTX in isotonic buffer at pH 7.4 and 37 degrees C for 7 days.

RESULTS

The G-MTX and M-GEL conjugates contained 21 and 1.2 mole MTX/mole gelatin and released 12 and 17% MTX by 7 days resulting in pseudo-first order release rate constants of 0.76x10(-3) and 1.0x10(-3) hr(-1), respectively. Alternate MTX species (< or =10%) were detected during the release study and were attributed to low molecular weight gelatin/MTX fragments and MTX polymers.

CONCLUSIONS

Gelatin/MTX conjugates having opposite conjugate bond polarities and minimal by-product crosslinking have been produced and slowly released MTX by hydrolytic cleavage indicating good stability for future cell culture studies.

Kinetics of hydrolysis of dextran-methylprednisolone succinate, a macromolecular prodrug of methylprednisolone, in rat blood and liver lysosomes

A macromolecular prodrug of methylprednisolone (MP) was synthesized by conjugating MP with dextran with a M(W) of 70000 through a succinic acid linker. It has been shown previously that the dextran-MP conjugate (DMP) releases MP directly or indirectly through formation of methylprednisolone succinate (MPS) which is further hydrolyzed to MP. To investigate the suitability of DMP conjugate as a prodrug of MP for systemic administration, the kinetics of hydrolysis of the conjugate was studied in vitro in rat blood and liver lysosomes. In blood, the hydrolysis of MPS to MP was approximately ten-fold faster than that in buffer. However, the hydrolysis rate constants of DMP conjugate to MP or MPS in blood were not different from those in buffer. Overall, the hydrolysis of DMP in the rat blood occurred with a half life of approximately 25 h. Hydrolysis of MPS to MP also occurred in the liver lysosomal fraction, but not in the control samples lacking lysosomes. However, the rate constants for the hydrolysis of DMP conjugate to MP and MPS in the lysosomal fraction were not significantly different from those in the control samples. These data suggest that the slow hydrolysis of DMP conjugate to MP or MPS in both rat blood and liver lysosomes occurs mostly, if not completely, via chemical hydrolysis. However, the conversion of MPS to MP is apparently enzymatic. The data may have significant implications for systemic administration of the prodrug.

High-performance size-exclusion chromatographic analysis of dextran-methylprednisolone hemisuccinate in rat plasma

A size exclusion chromatographic method is presented for the measurement of the concentrations of a macromolecular prodrug of methylprednisolone (MP), dextran-methylprednisolone succinate (DEX-MPS), in rat plasma. After precipitation of the plasma (100 microl) proteins with perchloric acid, the samples are injected into a size exclusion column with a mobile phase of water:acetonitrile:glacial acetic acid (75:25:0.2) and a flow-rate of 1 ml/min. The DEX-MPS conjugate, detected at 250 nm, elutes at a retention time of approximately 6.5 min, free of endogenous peaks. Excellent linear relationships (r2=0.997) were found between the detector response and the concentrations of DEX-MPS in the range of 2-100 microg/ml (MP equivalent), with intra- and inter-run C.V.s of <6% and error values of <5%. The application of the assay was also demonstrated by measurement of the plasma concentrations of DEX-MPS after single 5 or 10 mg/kg doses of the conjugate administered intravenously to rats.

Pharmaceutical perspectives of nonviral gene therapy

The use of nonviral plasmid-based gene medicines represents an attractive in vivo gene transfer strategy that is simple and lacks many risks that are inherent to viral systems. Commercialization of gene medicines requires a thorough analysis of business opportunities, unmet clinical needs, competitive products under development, and issues related to intellectual property. Synthetic gene delivery systems are designed to control the location of a gene within the body by affecting distribution and access of a gene expression system to the target cell, and/or recognition by a cell surface receptor and uptake followed by intracellular and nuclear translocation. Plasmid-based gene expression systems are designed to control the level, fidelity, and duration of in vivo production of a therapeutic gene product. This review will provide insights into the potentials of plasmid-based gene therapy and critical evaluation of gene delivery sciences and clinical applications of gene medicines.

Synthesis and pharmacokinetics of a novel macromolecular prodrug of Tacrolimus (FK506), FK506-dextran conjugate

A novel macromolecular prodrug of Tacrolimus (FK506), FK506-dextran conjugate, was developed and its physico-chemical, biological and pharmacokinetic characteristics were studied. The conjugate was estimated to contain 0.45% of FK506 and the coupling molar ratio was approximately 1:1 (dextran-FK-506). Adsorption experiments using ion exchangers indicated that FK506-dextran conjugate acted as a weakly negatively charged macromolecule. Low molecular weight radioactive compound(s), which was eluted in the same fractions as [(3)H]FK506, was released from [(3)H]FK506-dextran conjugate by chemical hydrolysis with a half-life of 150 h in phosphate buffer. In vitro immunosuppressive activity of the conjugate, as assessed by the rat lymphocyte stimulation test, was almost comparable to that of free FK506, suggesting that biologically active FK506 could be liberated from the conjugate. In vitro biodistribution studies demonstrated that conjugation with the dextran derivative dramatically changed the pharmacokinetic properties of FK506 after intravenous injection in rats. AUC of the FK506-dextran conjugate was almost 2000 times higher than that of free FK506 and organ uptake clearances of the conjugate were significantly smaller than those of the free drug. Thus, the present study has demonstrated that the FK506-dextran conjugate behaves as a prodrug of FK506 with an extended blood circulating time and can be expected to have an improved therapeutic potency.

Extravasation of macromolecules

Macromolecules can extravasate across the normal endothelium by transcapillary pinocytosis as well as by passage through interendothelial cell junctions, gaps or fenestrae. The main biological factors that control extravasation of a solute include regional differences in the capillary structures, the disease state of the organ or tissue, and the rate of blood and lymph supply. Physicochemical properties that are of profound significance in the extravasation of macromolecules are molecular size, shape, charge and hydrophilic/lipophilic balance (HLB) characteristics. Extravasation of small drugs, proteins, oligonucleotides and genes can be controlled by conjugating or forming complexes with macromolecular carriers. This requires a thorough understanding of the relationship between the chemical structures, physicochemical properties and the pharmacokinetics of both carrier and active molecules. This review article discusses the extravasation of macromolecules from the view points of pharmacokinetics and drug delivery systems, with the main emphasis on the extravasation across the liver, kidney and tumor capillaries.

Antitumor activity of methotrexate-albumin conjugates in rats bearing a Walker-256 carcinoma

We have recently reported that albumin accumulates in solid tumors and serves there as a source of nitrogen and energy. Methotrexate-albumin conjugates [MTX(I)-RSA] derivatized at a molar ratio of 1:1 differ favorably from original MTX in terms of plasma presence and tumor uptake. The purpose of this study was to evaluate the therapeutic efficacy of these novel conjugates in a comparative study with low m.w. MTX is Sprague-Dawley rats bearing a Walker-256 carcinoma. The maximum tolerated dose (MTD) for MTX and MTX(I)-RSA was determined (2 mg/kg based on MTX injected on days 1, 3 and 8). The tumor-bearing rats received injections of either the MTD or MTD/2 of MTX, MTX-albumin or mixtures containing the MTD/2 or MTD/4 of both MTX and MTX-albumin. No toxic side effects were observed. Cure rate and tumor growth retardation were slightly better for the conjugate compared with MTX alone in the MTD group (16 complete remissions vs. 14 of 20 rats). The best results were achieved for the combination treatment with MTX and MTX-albumin, with complete remission in all 20 rats. In conclusion, MTX-albumin conjugates show therapeutic activity in vivo without toxic side effects. Additive effects were observed for a combination of MTX-albumin and MTX. These effects might be caused by the much longer tumor exposition time of the conjugate in conjunction with a different route of uptake (pinocytosis for MTX-albumin vs. folate receptors for MTX).

Pharmacokinetic characteristics and therapeutic effects of mitomycin C-dextran conjugates after intratumoural injection

The pharmacokinetics and therapeutic effects of macromolecular prodrugs of mitomycin C (MMC), MMC-dextran conjugates (MMC-D) were studied after intratumoural injection in rats bearing Walker 256 carcinosarcoma. As the first step, the intratumoural disposition characteristics of these drugs were delineated in perfusion experiments employing a tissue-isolated tumour preparation. While MMC immediately disappeared from the tumour preparation following direct intratumoural injection, cationic and anionic MMC-D were retained in the tumour longer, demonstrating that the intratumoural clearance of MMC can be greatly retarded by dextran conjugation. The effect was more pronounced in the case of the cationic conjugate. Venous outflow data in the perfusion experiments were analyzed based on a compartment model in which the tumour tissue was assumed to consist of two compartments, one well- and the other poorly-perfused. The pharmacokinetic analysis revealed that macromolecular conjugation reduced elimination of MMC from the poorly-perfused region rather than well-perfused region. Simulation of conjugated and free MMC levels in the tissue using the calculated parameters clearly showed that intratumoural injection of MMC-D, especially the cationic form, can maintain a certain level of active free MMC in the tissue for a much longer time period. The long retention of cationic MMC-D in tumour after intratumoural injection was also confirmed by an in vivo pharmacokinetic study and whole body autoradiography in rats bearing subcutaneous Walker 256 carcinosarcoma. In addition, superior antitumour activity of cationic MMC-D was observed against subcutaneous tumours after intratumoural injection. Together with the finding that MMC is selectively toxic to hypoxic tumour cells at low concentrations, these pharmacokinetic studies strongly support the therapeutic efficacy of the macromolecular prodrugs.

Transferrin conjugates of doxorubicin: synthesis, characterization, cellular uptake, and in vitro efficacy

One strategy for improving the antitumor selectivity and toxicity profile of antitumor agents is to design drug carrier systems employing suitable carrier proteins. Thus, thiolated human serum transferrin was conjugated with four maleimide derivatives of doxorubicin that differed in the stability of the chemical link between drug and spacer. Of the maleimide derivatives, 3-maleimidobenzoic or 4-maleimidophenylacetic acid was bound to the 3'-amino position of doxorubicin through a benzoyl or phenylacetyl amide bond, and 3-maleimidobenzoic acid hydrazide or 4-maleimidophenylacetic acid hydrazide was bound to the 13-keto position through a benzoyl hydrazone or phenylacetyl hydrazone bond. The acid-sensitive transferrin conjugates prepared with the carboxylic hydrazone doxorubicin derivatives exhibited an inhibitory efficacy in the MDA-MB-468 breast cancer cell line and U937 leukemia cell line comparable to that of the free drug (employing the BrdU (5-bromo-2'-deoxyuridine) incorporation assay and tritiated thymidine incorporation assay, respectively, IC50 approximately 0.1-1 mM), whereas conjugates with the amide derivatives showed no activity. Furthermore, antiproliferative activity of the most active transferrin conjugate (i.e. the conjugate containing a benzoyl hydrazone link) was demonstrated in the LXFL 529 lung carcinoma cell line employing a sulforhodamine B assay. In contrast to in vitro studies in tumor cells, cell culture experiments performed with human endothelial cells (HUVEC) showed that the acid-sensitive transferrin conjugates of doxorubicin were significantly less active than free doxorubicin (IC50 values approximately 10-40 higher by the BrdU incorporation assay), indicating selectivity of the doxorubicin-transferrin conjugates for tumor cells. Fluorescence microscopy studies in the MDA-MB-468 breast cancer cell showed that free doxorubicin accumulates in the cell nucleus, whereas doxorubicin of the transferrin conjugates is found localized primarily in the cytoplasm. The differences in the intracellular distribution between transferrin-doxorubicin conjugates and doxorubicin were confirmed by laser scanning confocal microscopy in LXFL 529 cells after a 24 h incubation that revealed an uptake and mode of action other than intercalation with DNA. The relationship between stability, cellular uptake, and cytotoxicity of the conjugates is discussed.

Low toxicity and high antitumour activity of daunomycin by conjugation to an immunopotential amphoteric branched polypeptide

The acid labile derivative of Daunomycin cis-aconityl Daunomycin (cAD), was coupled to an amphoteric polypeptide, poly[Lys-(Glui-DL-Alam)] (EAK), which was selected for conjugation on the basis of its pharmacological and immunological properties. The systemic toxicity of covalently attached Daunomycin was studied by monitoring body weight, life-span, bone marrow and haematological parameters of BDF1 mice. More than 3-fold the lethal dose of free Daunomycin could be applied without serious toxic effect when the drug was attached to EAK. The dose- and time-dependent modulatory effect of free drug and [cAD]-EAK conjugate on the humoral and cellular immune response to sheep red blood cell antigens in mice was studied. The conjugation of Daunomycin to EAK carrier polypeptide compensated for the immunosuppression induced by free Daunomycin. [cAD]-EAK conjugate at Daunomycin doses of 2-10 mg/kg was very effective against L1210 leukaemia producing 66-100% long-term survivors (> 60 days), while Daunomycin in itself increased the mean survival only by 52%, with no long-term survivors. The mixture of free Daunomycin and EAK polypeptide had similar toxicity and antitumour activity as free Daunomycin, indicating the important role of covalent attachment in increased therapeutic efficacy.

Effect of particle size and charge on the disposition of lipid carriers after intratumoral injection into tissue-isolated tumors

PURPOSE

Pharmacokinetic properties of various lipid carriers (liposome and emulsions) after intratumoral injection were studied in perfusion experiments using tissue-isolated tumor preparations of Walker 256 carcinosarcoma.

METHODS

Four types of lipid carriers, large emulsion (254 nm), small emulsion (85 nm), neutral liposomes (120 nm) and cationic liposomes (125 nm) were prepared. We quantified their recovery from the tumor, leakage from the tumor surface and venous outflow after intratumoral injection into perfused tissue-isolated tumors, and analyzed venous appearance curves based on a pharmacokinetic model.

RESULTS

In contrast to the small emulsion and neutral liposomes, which immediately appeared in the venous outflow perfusate following intratumoral injection, the appearance of the cationic liposomes and the large emulsion was highly restricted, clearly demonstrating that intratumoral clearance of these formulations can be greatly retarded by the cationic charge and large particle size, respectively. The venous appearance rate-time profiles were fitted to equations derived from a two-compartment model by nonlinear regression analysis. When the calculated parameters were compared among these four formulations, the venous appearance rate did not exhibit such a large difference; however, the rate of transfer from the injected site to the compartment which involves clearance by venous outflow was all very different.

CONCLUSIONS

The results of this study indicate that the determining factor which alters the pharmacokinetic properties of these lipid carriers after intratumoral injection is not the rate of transfer from the interstitial space to the vascular side but the rate of intratumoral transfer from the injection site to the well-vascularized region.

Pharmacokinetic analysis of drug disposition after intratumoral injection in a tissue-isolated tumor perfusion system

PURPOSE

The purpose of this study was to establish an experimental system for evaluation of the intratumoral behavior of drugs after intratumoral injection using perfused tissue-isolated tumor preparations of Walker 256 carcinoma (3.46-9.73g, n = 16).

METHODS

We quantified the recovery of Phenol Red (model drug) in the tumor, leakage from the tumor surface and the venous outflow after intratumoral injection using perfused tissue-isolated tumors, and analyzed venous appearance curves based on a pharmacokinetic model in which the tumor tissue was assumed to be divided into two compartments, i.e., well- and poorly-perfused regions.

RESULTS

In small tumors (Type 1, 5.42 +/- 0.39 g), the drug appeared immediately in the venous outflow, and the amount remaining in the tumor tissue at 2 hr after injection was small. In contrast the venous appearance rate reached a significantly lower peak a few minutes after injection, and a large amount of injected drug remained in some large tumors (Type 2.8.17 +/- 0.51 g). Pharmacokinetic analysis revealed that there was a correlation between tumor weight and the rate constants of transfer from the poorly-perfused region to the well-perfused region, and between the rate constants of transfer from the well-perfused region to the venous outflow and dosing ratios into the well-perfused region.

CONCLUSIONS

An experimental system and analytical method were established for the evaluation of the intratumoral behavior of drugs after intratumoral injection using a tissue-isolated tumor perfusion system. This experimental system will be useful in analyzing the antitumor drug disposition after intratumoral injection.

Development and pharmacokinetics of galactosylated poly-L-glutamic acid as a biodegradable carrier for liver-specific drug delivery

PURPOSE

A biodegradable carrier for the liver-specific delivery of drugs was developed using poly-L-glutamic acid (PLGA) modified with galactose (galactosylated PLGA or Gal-PLGA), and its feasibility was investigated in mice.

METHODS

111In-PLGA and 111In-Gal-PLGAs were injected in mice and their distribution and biodegradation properties were studied.

RESULTS

After intravenous injection. 111In-PLGA was rapidly eliminated from the plasma and recovered mainly in the kidneys and urine. Approximately 15% of the dose was recovered in the liver, predominantly in the nonparenchymal cells. 111In-Gal-PLGAs were taken up by the liver parenchymal cells. Derivatives having 16 or more galactose residues were taken up by the liver to a higher extent (> 60% of the dose). The hepatic clearance of 111In-Gal-PLGAs correlated with their number of galactose residues. 111In-Gal18-PLGA was degraded into low-molecular weight products in the liver.

CONCLUSIONS

The advantageous in vivo properties of Gal-PLGA as a liver-specific biodegradable carrier of drugs were demonstrated in mice.

Macromolecular carrier systems for targeted drug delivery: pharmacokinetic considerations on biodistribution

This review article describes the current status and future perspectives of site-specific drug delivery by means of macromolecular carrier systems. Basic aspects and recent advances of targeted delivery of 1) conventional drugs, 2) protein drugs, and 3) gene medicines including antisense oligonucleotides and plasmid DNA, are reviewed from a pharmacokinetic perspective. Successful in vivo application of macromolecular carrier systems requires pharmacokinetic considerations at whole body, organ, cellular and subcellular levels. The integration of simultaneous research progress in the multidisciplinary fields such as biochemistry, cell and molecular biology, pharmacology, and pharmacokinetics will accelerate the emergence of marketed drugs with macromolecular carrier systems.

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

Antitumor drugs can be classified into two groups; cell cycle phase nonspecific (type I) and specific (type II) drugs. The cytotoxic activity of type I drugs depends on the time-concentration product (AUC), whereas that of type II drugs is time-dependent. Therefore, not only the AUC in the target organ, but also the exposure time is an important factor for evaluating the efficiency of any delivery system for antitumor drugs. In the present study, we examined the factors governing the cytotoxicity of drugs in tumors based on a hybrid perfusion model. It is suggested that the increase in tumor tissue binding of drug results in an increased unbound drug mean residence time (MRTT,U), leading to the increased activity of type II drugs. In contrast, the cytotoxic activity of type I drugs is unaffected by the alteration in the tissue binding since the intracellular AUC defined for unbound drugs (AUCT,U) is unaffected by the extent of drug binding. We also found that the symmetrical increase in the permeability-surface area products (PS) for drug influx (PSinf) and efflux (PSeff) across the tumor plasma membrane results in the unaltered and reduced antitumor activity for the type I and type II drugs, respectively, due to the unaltered AUCT,U and to the reduced MRTT,U. The kinetic analysis suggests that the increase in PSinf/PSeff ratio results in the increased cytotoxic activity of both type I and type II drugs. Collectively, optimization of the antitumor activity can be attained by increasing the tissue binding for type II drugs and by increasing PSinf and/or by decreasing PSeff type I and type II drugs. The present simulation study was carried out by considering the pharmacodynamic features of antitumor drugs and was a method of predicting how the antitumor activity may change on altering each process (tissue binding and membrane permeability for the influx and efflux processes) which governs the characteristics of drug distribution to tumors.