Inhibition of [3H]thymidine incorporation into DNA of rat regenerating liver by 2',2'-difluorodeoxycytidine coupled to lactosaminated poly-L-lysine

Albumin-drug conjugates in the treatment of hepatic disorders

INTRODUCTION

This review deals with the use of serum albumin (SA) as a carrier for the selective delivery of drugs to liver cells.

AREAS COVERED

The synthesis and properties of the SA conjugates prepared to enhance the performance of the drugs used in the treatment of viral hepatitis, hepatocellular carcinoma (HCC), liver micrometastases and hepatic fibrosis are reported.

EXPERT OPINION

Studies in humans and laboratory animals demonstrated the capacity of SA conjugates to accomplish a liver targeting of the drugs, but at the same time underscored their limits and drawbacks, which can explain why to date these complexes did not reach a practical application. The major drawback is the need of administration by intravenous route, which prevents long-term daily treatments as required by some liver pathologies, such as chronic virus hepatitis and fibrosis. At present, only a conjugate carrying doxorubicin and addressed to the treatment of HCC showed in laboratory animals a solid potentiality to improve the value of the coupled drug. In the future, conjugation to SA could remain a successful strategy to permit the administration of drugs with rapid resolutive effects inside liver cells without causing severe extrahepatic adverse reactions.

Lactosaminated human albumin, a hepatotropic carrier of drugs

A selective delivery of drugs to liver can be obtained by conjugation with galactosyl terminating macromolecules. The conjugates selectively enter hepatocytes after interaction of the carrier galactose residues with the asialoglycoprotein receptor (ASGP-R) present only on these cells. Within hepatocytes the conjugates are transported to lysosomes where the drug is set free from the carrier, becoming concentrated in liver cells. The present article reviews the liver targeting of drugs obtained with lactosaminated albumin (L-SA), a neoglycoprotein exposing galactosyl residues. We report: (1) experiments which demonstrate the antiviral efficacy of the L-H(human)SA-ara-AMP conjugate in laboratory animals and in humans with viral hepatitis; (2) the property of a L-HSA conjugate with fluorodeoxyuridine to produce concentrations of the drug higher in hepatic sinusoids than in systemic circulation, with the potential of accomplishing a loco-regional, noninvasive treatment of liver micrometastases; (3) the increased anticancer activity of doxorubicin (DOXO) when coupled to L-HSA on all the forms of chemically induced rat hepatocellular carcinomas including those which do not express the ASGP-R.

Doxorubicin coupled to lactosaminated human albumin: a hepatocellular carcinoma targeted drug

Hepatocellular carcinoma (HCC) is the fifth most common malignancy worldwide. There is a substantial need for new chemotherapeutic drugs effective against this tumor. Doxorubicin (DOXO), used for chemoembolization of HCCs, is poorly efficacious when administered systemically at conventional doses; dose escalation is hindered by unacceptable toxicity. Here, we review preclinical experiments showing that the efficacy of DOXO against HCCs and its safety increased following conjugation to lactosaminated human albumin (L-HSA). L-HSA-DOXO was initially prepared to improve the anticancer activity of the drug on well-differentiated HCCs, which actively internalize L-HSA by means of the asialoglycoprotein receptor. Unexpectedly, it was found that the conjugate enhanced DOXO concentrations in all forms of HCCs, independently of their differentiation grade.

Doxorubicin coupled to lactosaminated albumin inhibits the growth of hepatocellular carcinomas induced in rats by diethylnitrosamine

BACKGROUND/AIMS

The hepatocyte receptor for asialoglycoproteins internalizes galactosyl terminating macromolecules which can be used as hepatotropic drug carriers. Since this receptor is also expressed on the cells of well differentiated human hepatocellular carcinomas (HCCs), we studied whether conjugation of doxorubicin (DOXO) with lactosaminated human albumin (L-HSA) increases the drug efficacy on HCCs induced in rats by diethylnitrosamine (DENA).

METHODS

DENA was given in the drinking water for 8 weeks. One week after the last day of DENA administration, animals were randomly assigned to three groups. Each group was administered with either saline, free or coupled DOXO (1 microg/g). Rats received 4 weekly intravenous injections. One week after the last administration, rats were killed and HCC development was evaluated by counting the tumor nodules on the surface of hepatic lobes.

RESULTS

In rats treated with L-HSA coupled DOXO the number of neoplastic nodules was significantly lower (P < 0.05) than that counted in animals injected with saline or with free DOXO. Coupled DOXO did not decrease body rat weight, which was markedly reduced by the free drug.

CONCLUSIONS

Conjugation with L-HSA increased the antineoplastic efficacy and decreased the systemic toxicity of DOXO administered to rats with HCCs produced by DENA.

Enhanced uptake of lactosaminated human albumin by rat hepatocarcinomas: implications for an improved chemotherapy of primary liver tumors

BACKGROUND/AIMS

The hepatocyte receptor for asialoglycoproteins (ASGP-R) internalizes macromolecules exposing galactosyl residues (MEGRs) which can be used as liver-addressed drug carriers. This receptor was also found on the cells of the large majority of well differentiated hepatocarcinomas (HCCs). The aim of the present experiments was to ascertain whether ASGP-R of HCCs is functionally active and these tumors can internalize higher quantities of MEGRs than extra-hepatic tissues.

METHODS

We injected radioactive lactosaminated human albumin (L-HSA) in rats with HCCs produced by nitroso-diethylamine and measured the radioactivity of tumors, surrounding liver, heart, intestine and kidney. L-HSA is a MEGR successfully used in humans as a hepatotropic drug carrier.

RESULTS

The levels of radioactivity of HCCs were two to three times lower than those of surrounding liver, but several times higher than those of extra-hepatic tissues. L-HSA accumulation in the tumors mainly occurred via the ASGP-R, as indicated by the 20 times lower penetration of non-lactosaminated HSA. L-HSA uptake by the well-differentiated tumors were four times higher compared with that by the poorly differentiated forms.

CONCLUSIONS

The present results suggest that in the chemotherapy of HCCs expressing the ASGP-R the extra-hepatic toxicity of anticancer agents can be reduced by conjugation to L-HSA.

A novel method for coupling doxorubicin to lactosaminated human albumin by an acid sensitive hydrazone bond: synthesis, characterization and preliminary biological properties of the conjugate

The expression of the asialoglycoprotein receptor on the cells of the large majority of the well differentiated hepatocellular carcinomas can be exploited to improve the chemotherapy of these tumours by coupling anticancer agents to macromolecules taken up by the receptor. In line with this approach, in previous experiments we coupled doxorubicin (DOXO) to lactosaminated human albumin (L-HSA) using the (6-maleimidocaproyl)hydrazone derivative of the drug as an acid sensitive linker. Encouraging results were obtained in laboratory animals using L-HSA-DOXO. This conjugate, however, has the disadvantage of a difficult synthesis, which requires protein thiolation with iminothiolane and can hinder its preparation on a large scale. Here we describe a very simple method of coupling. The HS-groups required for the reaction with the maleimide moiety of DOXO-EMCH are made available in L-HSA by a cleavage of the protein disulphides achieved with tris(2-carboxyethyl) phosphine (TCEP). Contrary to thiolic reducing agents, the use of TCEP eliminates the need of an inert atmosphere and allows a one-step coupling reaction, without purification of the reduced protein before the addition of DOXO-EMCH. As the previous L-HSA-DOXO conjugate, the new conjugate accomplishes a very efficient liver targeting of the drug. This novel method of synthesis should facilitate the preparation of L-HSA-DOXO in the amounts required for clinical studies.

Doxorubicin coupled to lactosaminated human albumin remains confined within mouse liver cells after the intracellular release from the carrier

BACKGROUND

The hepatocyte receptor for asialoglycoproteins, which binds and internalises galactosyl terminating peptides, was found to be expressed also on the cells of the majority of hepatocarcinomas.

AIMS

To verify whether doxorubicin coupling to lactosaminated albumin, a galactosyl terminating neoglycoprotein, produces selective drug accumulation in hepatocytes with reduced concentrations in extra-hepatic tissues, thus facilitating the use of the drug in hepatocarcinoma treatment.

METHODS

Doxorubicin concentrations were measured in organs of mice injected with the free or coupled drug.

RESULTS

In mice injected with the coupled drug, the ratios between doxorubicin concentrations in liver and those in heart, intestine, spleen and kidney were 8-14 times higher than in animals that received the same dose of the free drug.

CONCLUSIONS

Due to the very efficient liver targeting of doxorubicin, the lactosaminated human albumin-doxorubicin conjugate appears to have the potential of improving the chemotherapy of hepatocellular carcinomas through the asialoglycoprotein receptor.

Floxuridine coupling with lactosaminated human albumin to increase drug efficacy on liver micrometastases

BACKGROUND

Conjugates of nucleoside analogues with galactosyl terminating peptides selectively enter hepatocytes through the asialoglycoprotein receptor. After intracellular release from the carrier, the drugs partly exit from hepatic cells into hepatic blood.

AIMS

To establish whether administration of a conjugate of floxuridine with lactosaminated human albumin selectively enhances drug concentrations in hepatic blood. Floxuridine is a fluoropyrimidine active on human colorectal cancer, a tumour which metastasises first to the liver.

METHODS

In rats injected with free or conjugated floxuridine, plasma levels of the drug were determined in hepatic veins and in inferior vena cava, in order to measure drug concentrations in hepatic blood and in the systemic circulation, respectively.

RESULTS

Ratios between floxuridine levels in hepatic veins and those in systemic circulation were found to be seven times higher in rats injected with the conjugate (p=0.000).

CONCLUSIONS

The present results suggest that coupling to lactosaminated albumin might improve the effect of floxuridine in adjuvant chemotherapy of colorectal cancer by exposing the cells of liver micrometastases (nourished by hepatic sinusoids) to enhanced drug concentrations.

Conjugates of nucleoside analogs with lactosaminated human albumin to selectively increase the drug levels in liver blood: requirements for a regional chemotherapy

Nucleoside analogs (NAs) conjugated with galactosyl terminating peptides selectively enter hepatocytes via the asialoglycoprotein receptor and, after intracellular release from the carrier, partly exit from these cells into the bloodstream, resulting in higher concentrations in liver blood than in systemic circulation. Therefore, conjugates of anticancer NAs can be exploited to accomplish a loco-regional noninvasive treatment of liver micrometastases. In the present experiments we studied whether the enhancement of drug levels in liver blood achieved when NAs are given in the coupled form depends on the rate of drug elimination from the bloodstream. Three NAs, adenine arabinoside (ara-A), 5-fluoro-2'-deoxyuridine (FUdR), and 2',2'-difluorodeoxycytidine, were coupled with lactosaminated human albumin, a galactosyl terminating carrier. In rats that received an intravenous bolus injection of these conjugates, we compared the drug concentrations in liver blood to those in the systemic circulation. We found that enhanced levels of NAs in liver blood were only achieved by administering the conjugates of the drugs (ara-A and FUdR), which are rapidly cleared from the bloodstream. Increased drug levels also were obtained when ara-A and FUdR conjugates were slowly infused (a way of administration often used for anticancer drugs). The experiments also showed that galactosyl terminating conjugates of NAs might have the potential to produce a therapeutic effect only when the coupled drugs are active at low blood concentrations, since the amounts of drugs introduced into hepatocytes and released by these cells in the bloodstream cannot be increased when the receptor for the hepatic uptake of galactosyl terminating peptides is saturated.

Coupling of 5-fluoro 2'-deoxyuridine to lactosaminated poly-l-lysine: an approach to a regional, non-invasive chemotherapy of liver micrometastases

Nucleoside analogs conjugated with galactosyl-terminating peptides selectively enter liver cells and after intracellular release from the carrier partly exit into bloodstream, resulting in higher concentrations in liver blood than in systemic circulation. The aim of the present experiments was to ascertain whether, in mice injected with non-toxic doses of a 5-fluoro 2'-deoxyuridine (FUdR) conjugate with lactosaminated poly-L-lysine (L-poly(LYS)), the drug was released by hepatic cells in high enough amounts to be pharmacologically active on neoplastic cells infiltrating the liver. We observed that L-poly(LYS)-FUdR inhibited the growth of hepatic metastases induced by intrasplenic administration of murine colon carcinoma C-26 cells. L-poly(LYS)-FUdR was not toxic for C-26 cells in vitro, was selectively taken up by mouse liver, and was stable in mouse blood, indicating that the effect on the metastases was due to FUdR (and/or its active metabolites) released in liver blood after the conjugate was taken up by the hepatic cells. These results suggest that L-poly(LYS)-FUdR might be useful in adjuvant chemotherapy of tumors giving liver metastases. The drug released from hepatic cells into liver blood following conjugate administration via the peripheral venous route might accomplish a locoregional, non-invasive treatment of micrometastases nourished by liver sinusoids.

Enhanced liver blood concentrations of adenine arabinoside accomplished by lactosaminated poly-L-lysine coupling: implications for regional chemotherapy of hepatic micrometastases

Conjugates of antiviral and antiblastic nucleoside analogs (NAs) with galactosyl-terminating peptides selectively enter hepatocytes after binding of the carrier galactose residues to the asialoglycoprotein receptor. Since NAs, when set free from the carrier within hepatocytes, partly exit from these cells into the bloodstream, we considered the possibility that administration of galactosyl-terminating conjugates of NAs could result in plasma concentrations of these drugs that would be higher in liver sinusoids than in capillaries of other organs. In the present study we demonstrated the validity of this hypothesis. We injected rats with a conjugate of adenine arabinoside (ara-A) with lactosaminated poly-L-lysine and found that the plasma concentrations of ara-A were >2-fold higher in blood of liver than in systemic circulation. Liver blood was collected from the inferior vena cava after closing below and above the outflows of the hepatic veins. The present result suggests that conjugation with galactosyl-terminating peptides might be a way to selectively increase the concentrations of NAs not only in hepatocytes, which have the asialoglycoprotein receptor, but also in cells infiltrating the liver, such as neoplastic cells of micrometastases nourished by hepatic sinusoids.