Introduction

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The pharmacological treatment of brain diseases is often complicated by the inability of potent drugs to pass across the BBB, which is formed by the tight endothelial cell junctions of capillaries within the brain. Malignant brain tumors respond poorly to chemotherapy because most of the anticancer agents cannot be delivered efficiently to the tumor site. For example the anthracycline doxorubicin, one of the most powerful anticancer agents, cannot be delivered systemically in effective concentrations to the tumor cells because of multidrug-resistance mechanisms (Ohnishi et al., 1995; Marbeuf-Gueye et al., 1997).

To overcome the limited access of drugs to the brain different methods have been developed that achieve BBB penetration (Temsamani et al., 2000). Some of these methods are characterized, for instance, by osmotic BBB opening or by the use of biologically active agents such as bradykinin (Kroll et al., 1998). Specific transport mechanisms have also been exploited, involving the activity of several independent transporters that mediate the influx of substances important for brain function, such as monocarboxylic acid, and amine and neutral amino acid transporters (Terasaki and Tsuji, 1994). A further strategy has been to incorporate the therapeutic drugs within delivering devices such as nanoparticles and liposomes (Huwyler et al., 1996; Gulyaev et al., 1999) or to conjugate the drug with a protein (Singh, 1999), a peptide vector (Pardridge et al., 1994; Schwarze et al., 1999), or with an antibody (Pardridge, 1999). This variety of strategies reflects the inherent difficulty in delivering drugs across the BBB.

Recently, we have shown that small peptide vectors can be used to enhance brain uptake of therapeutic drugs (Rousselle et al., 2000). These peptide vectors cross cellular membranes efficiently and have been used to enhance penetration of a number of drugs into live cells (Derossi et al., 1998). The SynB vectors are derived from natural peptides called protegrins (Harwig et al., 1995; Mangoni et al., 1996). They possess an amphipathic structure in which the positively charged and hydrophobic residues are separated in the sequence. They are able to cross efficiently cell membranes without any cytolytic effect. We have shown recently (Rousselle et al., 2000) by in situ rat brain perfusion and by intravenous injection in mice that coupling of doxorubicin to SynB1 vector enhances significantly its brain uptake. Additionally, the coupling led to a significant decrease of doxorubicin concentrations in the heart.

The mechanism by which these peptides cross the BBB is not known. Various peptides and proteins with a high permeability have been found to cross the BBB by several saturable mechanisms, including carrier-mediated transport of small peptides with an N-terminal tyrosine (Banks et al., 1986), receptor-mediated transcytosis of insulin (Duffy and Pardridge, 1987) or transferrin (Fishman et al., 1987), and adsorptive-mediated transcytosis of positively charged proteins, e.g., cationized albumin and -endorphin-cationized albumin chimeric peptide (Smith and Borchardt, 1989).

The aim of this study was to confirm the efficacy of the SynB vectors to deliver doxorubicin through the mouse BBB by the use of an in situ mouse brain perfusion model that we have recently developed (Dagenais et al., 2000) and to gain insight into the mechanism of transport. Complementary techniques were associated to it to measure the fraction of doxorubicin trapped into microvessel cells or present in brain parenchyma. We have used [¹⁴C]doxorubicin coupled to SynB1 (18 amino acids) as well as to a truncated derivative of SynB1: SynB3 (10 amino acids) and its enantio form D-SynB3. We show that coupling of doxorubicin to SynB vectors improves its penetration across the BBB with the same efficiency for all the SynB vectors used. We also demonstrate that the mechanism of transport of coupled doxorubicin involves a saturable system, which may operate via an adsorptive-mediated endocytosis.

	Materials and Methods

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Chemicals

[¹⁴C]Doxorubicin (specific activity = 55 Ci/mol) was purchased from Amersham Pharmacia Biotech (Orsay, France). [³H]Sucrose (specific activity = 10.20 Ci/mmol) was obtained from DuPont New England Nuclear (Paris, France). Doxorubicin hydrochloride, protamine, and poly(L-lysine) hydrobromide (up to mol. wt. 5000) were purchased from Sigma (St-Quentin, France). Peptide synthesis chemicals were obtained from Novabiochem (Läufelfingen, Switzerland). All other chemical and reagents were commercial products of reagent grade.

Preparation and Characterization of Peptide Conjugates

Peptide Synthesis. The peptides were assembled by conventional solid phase chemistry using a 9-fluorenylmethoxycarbonyl/tertiobutyl protection scheme (Atherton and Sheppard, 1989) and purified on preparative C18 reverse phase HPLC after trifluoroacetic acid (TFA) cleavage/deprotection. Purity of the lyophilized products was assessed by C18 reverse phase analytic HPLC and their molecular weight checked by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry. The peptides' sequences were SynB1 (RGGRLSYSRRRFSTSTGR, molecular mass, 2099 Da) L-SynB3 (RRLSYSRRRF, molecular mass, 1395 Da), and D-SynB3 (RRLSYSRRRF, the amino acids are in D form, molecular mass, 1395 Da).

Dox-SynB Synthesis (Fig. 1). Doxorubicin hydrochloride was suspended in dimethylformamide (DMF) containing diisopropylethylamine. Succinic anhydride (1 M equivalent; Fluka, Buchs, Switzerland) dissolved in DMF was added and incubated for 20 min. The resulting doxorubicin hemisuccinate was then activated by addition of benzotriazol-1-yl-oxopyrrolidinephosphonium hexafluorophosphate (1.1 M equivalent; Novabiochem, Läufelfingen, Switzerland) dissolved in DMF. The peptide was then added to the reaction mixture after 5 min of activation and left for another 20 min for coupling. Further processing and purity check of the conjugate was performed as described above. The molecular mass was found to be 2723 Da for dox-SynB1 and 2019 Da for both dox-SynB3 and dox-D-SynB3.

View larger version (14K): [in this window] [in a new window]   Fig. 1.   Structure of dox-SynB1 and dox-SynB3.

Radiolabeling of Dox-SynB. Preparations were performed as described above, except that [¹⁴C]doxorubicin was kept limiting by raising the stoichiometry of peptide, linkers, and activators to 1.3 M equivalent in the coupling reactions. The specific activity of dox-SynB1, dox-SynB3, and dox-D-SynB3 was 55 mCi/mmol, 2.04 Tbq/mol and the molar ratio of doxorubicin/peptide was 1:1. The radiochemical purity was estimated to be >98% according to the 480-nm chromatograms.

Distribution Coefficient Determination. The lipophilicity of the radiolabeled free and vectorized doxorubicin was estimated by measuring their partitioning between the perfusion buffer (pH = 7.4) and octan-1-ol. Distribution coefficients (D_{octanol/buffer pH = 7.4}) were determined at volume ratios of 1:1 by vigorously shaking the two phases together. The samples were then incubated at 37°C for 30 min to facilitate phase separation. One sample of each phase was weighed and the radioactivity counted in a gamma counter. D_{octanol/buffer pH = 7.4} was calculated as ([dpm/ml] in the octanol phase)/([dpm/ml] in the buffered-saline phase). Experiments were done in triplicate and the mean of the log D_{octanol/buffer pH = 7.4} was calculated.

Stability in Vitro. Dox-SynB solution (1 ml, 2 mg/ml) was mixed with 4 ml of mouse plasma (obtained from Iffa-Credo, L'Arbresle, France). At various times (0, 15, 25, 30, 40, 45, 60, 120, 180, 210 min), 250-µl aliquots were withdrawn and quenched in 1 ml of acid mixture (H₂O/TFA 0.1%). The vectorized doxorubicin and metabolites were then extracted from plasma by applying the sample on a C18 SPE cartridge and eluting in 500 µl of acetonitrile/isopropanol/H₂O/TFA (50:20:30:5, v/v) solution. The samples were then analyzed by HPLC at 418 nm on C18 column using acetonitrile/water gradient. The percentage of nondegraded vectorized doxorubicin and released doxorubicin was calculated.

Animals

Adult OF1 mice (30-40 g, 6-8 weeks old) were obtained from Iffa-Credo. Animals were maintained under standard conditions of temperature and lighting and had free access to food and water. The research adhered to the ethical rules of the French Ministry of Agriculture for experimentation with laboratory animals (Law No. 87-848).

In Situ Mouse Brain Perfusion Study

Surgical Procedure. The uptake of free or vectorized [¹⁴C]doxorubicin to the luminal side of mouse brain capillaries was measured using the in situ brain perfusion method previously adapted in our laboratory for the study of drug uptake in the mouse brain (Dagenais et al., 2000). Briefly, the right common carotid of ketamine/xylazine (140:8 mg/kg i.p.)-anesthetized mice was exposed and ligated at the heart side. The external carotid artery was ligated at the level of the bifurcation of the common carotid, rostral to the occipital artery. The common carotid was then catheterized rostrally with polyethylene tubing (0.30 mm i.d. × 0.70 mm o.d.; Biotrol Diagnostic, Chennevrières-les-Louvres, France) filled with heparin (25 U/ml) and mounted on a 26-gauge needle. The syringe containing the perfusion fluid was placed in an infusion pump (Harvard pump PHD 2000; Harvard Apparatus, Holliston, MA) and connected to the catheter. Immediately before the perfusion, the heart was stopped by severing the ventricles to eliminate contralateral blood flow contribution. Brains were perfused for 20 to 120 s at a flow rate of 2.5 ml/min. At the end of the perfusion time, the mouse was decapitated and the brain removed. The right hemisphere and samples of perfusion fluid were placed in preweighed scintillation vials and weighed. Brain and perfusion samples were then digested for 2 h in 1 ml of Solvable (Packard, Rungis, France) at 50°C and mixed with 9 ml of Ultima Gold XR scintillation cocktail (Packard). Total [¹⁴C] and [³H] were determined simultaneously in a Packard Tri-Carb model 1900 TR liquid scintillation analyzer and activities were converted from counts per minute to disintegrations per minute with the use of internally stored quenching curves.

Transport Studies of Radiolabeled [¹⁴C]Doxorubicin. The perfusate consisted of a Krebs-bicarbonate buffer: 128 mM NaCl, 24 mM NaHCO₃, 4.2 mM KCl, 2.4 mM NaH₂PO₄, 1.5 mM CaCl₂, 0.22 mM MgSO₄, and 9 mM D-glucose added before infusion. The solution was gassed with 95% O₂ and 5% CO₂ for pH control (=7.4) and warmed at 37°C in a water bath. Tracers were added to perfusate at concentrations of 0.4 µCi/ml for free doxorubicin, 0.1 µCi/ml for vectorized doxorubicin, and 0.3 µCi/ml for [³H]sucrose, a vascular marker that does not cross the BBB significantly. In some experiments, the brain uptake of [¹⁴C]dox-SynB was evaluated in the presence of various concentrations of unlabeled dox-SynB (0-100 µM). In other experiments we have studied the influence of polycationic peptides: poly(L-lysine) (0-25 µM) and protamine (25 µM) on the distribution of dox-SynB.

Determinations of BBB Transport Constants in the Right Mice Brain Hemisphere. Briefly, calculations were accomplished as previously described by Smith (1996). The integrity of the BBB was determined in each animal by the brain vascular volume (V_v, µl · g¹) estimated by the tissue distribution of [³H]sucrose from the following relationship:

(1)

where Q*_tot is the amount of radiolabeled sucrose in the right brain hemisphere (dpm · g¹) and C*_pf is the perfusate concentration of sucrose (dpm · µl¹).

(2)

1

(3)

1

(4)

1

1

(5)

(6)

1

1

1

Distribution in Brain Microvascular and Parenchymal Compartments. The distribution of [¹⁴C]doxorubicin between brain microvascular and parenchymal compartments was assessed using the capillary depletion method of Triguero et al. (1990) with some modifications (Rousselle et al., 2000). For this set of experiments, we used a dual-syringe infusion pump (Harvard Apparatus, Les Ulis, France) with one syringe containing the bicarbonate-buffered physiological saline with the radiotracer (syringe A) and the other without radiotracer (syringe B). The carotid catheter was connected to a four-way valve (Hamilton, Bonnaduz, Switzerland). After the carotid cannulation was completed and the appropriate connections were made, syringe A was discharged at a rate of 2.5 ml/min for 60 s. Syringe A was switched off and syringe B was switched on simultaneously to initiate the washout of the capillary space. After 60 s the mouse was decapitated and the right cerebral hemisphere was removed, weighed, and homogenized in 1 ml of capillary buffer (10 mM Hepes, 141 mM NaCl, 4 mM KCl, 1 mM NaH₂PO₄, 2.8 mM CaCl₂, 1 mM MgSO₄, and 10 mM D-glucose, pH = 7.4) on ice. After 15 strokes, 1 ml of a chilled 37% neutral dextran solution was added to obtain a final concentration of 18.5%. All homogenizations were performed at 4°C in a very short time. After taking an aliquot of homogenate, the solution was centrifuged at 3800g for 25 min at 4°C in a swinging-bucket rotor. The pellet and supernatant were carefully separated and counted in the liquid scintillation counter. The pellet was composed mainly of brain capillary and the supernatant reflected brain parenchyma.

(7)

Statistical Analysis. All experiments were performed on three to six mice. Data are expressed for the right cerebral hemisphere. Statistical comparisons conducted herein were accomplished by Student's unpaired t test or ANOVA. Bonferroni's multiple comparison test was used post hoc only when ANOVA results were significant. Statistical difference was accepted at the P < 0.05 significance level. Data are the mean ± S.E.M. Estimates of the dox-SynB transport parameters (K_m and V_max) were obtained by fitting a Michaelis-Menten-type equation to the uptake rate versus perfusate dox-SynB concentration data by nonlinear least-square regression using the SYSTAT software (SYSTAT, Inc., Evanston, IL).

	Results

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The radiolabeled model dox-peptides were shown to have a purity of at least 98%. To rule out that any degradation of the peptide vectors took place during the perfusion, we assessed the stability of the vectorized doxorubicin in mice plasma in vitro. Our results show that dox-SynB1 and dox-L-SynB3 have a degradation half-life of about 15 to 20 min in mice plasma, whereas for dox-D-SynB3, no significant degradation took place even after 48-h incubation in plasma (data not shown). Therefore, we assumed that no significant degradation of the compounds had taken place after 60 s of brain perfusion.

First, the brain uptake of [¹⁴C]doxorubicin in OF1 mice was studied by the in situ mouse brain perfusion method (Fig. 2). To choose a reliable perfusion time that allows a sufficient accumulation in the brain tissue, a time course study of brain distribution of doxorubicin was performed. The distribution volume (V_d) of doxorubicin to the brain was limited over 20 to 120 s but a sufficient accumulation of doxorubicin in the right brain hemisphere was reached after 60 s of perfusion, which was consequently the perfusion time selected for the following studies. For each mouse perfused, we have checked the integrity of the BBB by measuring the distribution volume of [³H]sucrose. Sucrose is commonly used as a marker of the integrity of the BBB because it does not measurably penetrate the BBB during short perfusion time. No differences in brain vascular volumes were observed between all perfusion times and the values measured were similar to those previously reported (Fig. 2).

View larger version (10K): [in this window] [in a new window]   Fig. 2.   The time-dependant uptake of [14C]doxorubicin () and [3H]sucrose () in the right brain hemisphere, expressed as apparent brain distribution volume measured with the in situ mouse brain perfusion method. Data are presented as means ± S.E.M. (n = 4-6 animals). For [14C]doxorubicin values have been corrected for [3H]sucrose/vascular space.

We then compared the brain uptake of free and coupled radiolabeled doxorubicin by measuring the total radioactivity in the right hemisphere of OF1 mice after 60 s of brain perfusion (Fig. 3A). When doxorubicin was coupled to SynB1, L-SynB3 (the amino acids are in the L form), or D-SynB3 (the amino acids are in the D form), a significant increase in brain uptake was obtained. An average of 30-fold increase in [¹⁴C]doxorubicin brain uptake was observed after its conjugation with either SynB1, L-SynB3, or D-SynB3 (Fig. 3A). It is noteworthy that no difference in brain uptake was observed between dox-L-SynB3 and dox-D-SynB3. This suggests that the mechanism of brain uptake does not involve a chiral receptor. To verify that this enhancement in brain uptake does not result from a loss in the integrity of the BBB, we simultaneously measured the distribution volume of [³H]sucrose (Fig. 3B). No differences in the sucrose vascular volumes were observed between all the tested compounds.

View larger version (42K): [in this window] [in a new window]   Fig. 3.   Distribution volumes of [14C]doxorubicin, [14C]dox-L-SynB1, [14C]dox-L-SynB3, and [14C]dox-D-SynB3 (A) and [3H]sucrose (B) in the right hemisphere of OF1 mice. The mice were perfused for 60 s with 3 µM doxorubicin, 0.5 µM dox-SynB1, dox-L-SynB3, and dox-D-SynB3. Each column represents a mean ± S.E.M. for n = 4 to 6 mice. Statistical significance is indicated by ***P < 0.001 using ANOVA followed by Bonferroni's test with the sucrose space subtracted.

To evaluate whether coupled doxorubicin has actually crossed the BBB or is simply trapped within brain endothelial cells, a washing procedure followed by the capillary depletion method of Triguero et al. (1990), which separates the whole brain into endothelial-enriched (pellet) and -depleted (supernatant) fractions, was performed. This procedure distinguishes between compounds remaining in the endothelial cells from those having crossed the abluminal endothelial membrane to enter the brain parenchyma. By this method, we observed that about half of the vectorized doxorubicin-derived radioactivity was associated with the parenchymal fraction after 60 s of perfusion followed by 60 s of washout (60.6 ± 7.7 and 44.3 ± 4.5% for dox-L-SynB1 and dox-L-SynB3, respectively) (Fig. 4).

View larger version (40K): [in this window] [in a new window]   Fig. 4.   Distribution rate of [14C]dox-L-SynB1 and [14C]dox-L-SynB3 in vascular pellet and supernatant fractions after washout and capillary depletion. Values are mean ± S.E.M. (n = 5 mice).

To explore the mechanism by which these peptide vectors cross the BBB, the uptake of [¹⁴C]dox-SynB was studied under conditions in which the brain was perfused with increasing concentrations of dox-SynB (0-100 µM). Figure 5 shows the brain uptake of [¹⁴C]dox-SynB1, [¹⁴C]dox-SynB3, and [¹⁴C]dox-D-SynB3 in the presence of various concentrations of unlabeled dox-SynB1, dox-SynB3, and dox-D-SynB3, respectively. Brain uptake of the three compounds after 60 s of perfusion was shown to be saturable. The K_m and V_max values are represented in Table 1. For example, the brain uptake of dox-D-SynB3 had a K_m of 9.0 µM and a V_max of 134 µmol · s¹ · g¹.

View larger version (18K): [in this window] [in a new window]   Fig. 5.   Rate of [14C]dox-SynB uptake in the right hemisphere as a function of dox-SynB concentration in perfusate after 60 s of perfusion. Values have been corrected for [3H]sucrose/vascular space. The solid line represents a fit of the Michaelis-Menten equation to the data by nonlinear least-square regression using the SYSTAT software (SYSTAT, Inc.).

To further shed light on the mechanism of transport, we used various inhibitors of endocytosis. Figure 6 shows the effect of increasing concentrations of poly(L-lysine) (0-25 µM) on the brain uptake of [¹⁴C]dox-D-SynB3. The concentration of dox-D-SynB3 used in this study was 0.5 µM and is lower that the half-saturation constant of the peptide transporter (K_m = 9.0 µM), which was thus not saturated. Addition of increasing concentrations of poly(L-lysine) led to a significant inhibition of [¹⁴C]dox-D-SynB3 brain uptake in a dose-dependent manner. The blood-brain transport of [¹⁴C]dox-D-SynB3 was also dramatically decreased by 25 µM protamine (Table 2). Similar results were obtained for [¹⁴C]dox-SynB1 and for [¹⁴C]dox-L-SynB3 (Table 2). During all experiments, the vascular volumes were monitored and found to be similar to those previously reported (Table 2).

View larger version (10K): [in this window] [in a new window]   Fig. 6.   Effects of various concentration of poly(L-lysine) on the uptake of [14C]dox-D-SynB3 by mouse brain. The murine brains were perfused for 60 s with 0.5 µM dox-D-SynB3 and increasing concentrations of poly(L-lysine) (0-25 µM). Values have been corrected for [3H]sucrose/vascular space. Each point represents the mean ± S.E.M. for n = 4 to 6 mice.

	Discussion

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The results of our experiments show that coupling doxorubicin with small peptide vectors results in delivery of this drug to the brain of experimental animals. Doxorubicin, despite a favorable partition coefficient (log D_{octanol/buffer, pH = 7.4} = 0.45 ± 0.06) failed to cross the BBB to an appreciable extent. Its brain uptake measured by the in situ mouse brain perfusion method is indeed very low (V_d = 28.2 ± 4.2 µl/s/g after 60 s of perfusion) and was similar to that obtained previously in rats (Rousselle et al., 2000). This low penetration may be related to the 170-kDa ATP-dependent efflux pump P-gp, which has been shown to be present at the luminal site of the endothelial cells of the BBB (Van Asperen et al., 1999). Our experiments using P-gp-deficient mice confirmed the efflux of doxorubicin by P-gp at the BBB because a higher K_in was obtained in this strain compared with normal mice (S. Cisternino, C. Rousselle, C. Dagenais, J. M. Lefauconnier, and J. M. Scherrmann, submitted). However, the efflux activity of P-gp may not be sufficient to explain the poor penetration of doxorubicin in relation to its chemical properties, even in the deficient mice. It is possible that other efflux pumps, such as the multidrug resistance-associated protein, may be responsible for the poor penetration of doxorubicin through the BBB. During in vitro studies using cancer cells, doxorubicin has been shown to be excluded by multidrug resistance-associated protein (Abe et al., 1994; Marbeuf-Gueye et al., 1997).

Coupling doxorubicin with SynB vectors resulted in a significant enhancement of its brain uptake although the dox-SynB compounds are less lipophilic (log D_{octanol/buffer, pH = 7.4} = 1.44 for dox-SynB1, 1.27 for dox-L-SynB3, and 1.30 for dox-D-SynB3). An average of 30-fold increase was obtained after coupling the doxorubicin with SynB1, L-SynB3, or D-SynB3. These results confirmed the efficacy of SynB vectors in delivering doxorubicin into the brain without compromising the integrity of the tight junctions. It is noteworthy that SynB3, a truncated derivative of SynB1, gave similar enhancement of doxorubicin brain uptake as SynB1.

To demonstrate that vectorized doxorubicin is not trapped inside the endothelial cells but has actually crossed the BBB, we carried out the wash-out procedure and the capillary depletion method. Our results indicate that about 50% of doxorubicin-derived radioactivity was associated with the parenchyma after 60 s of brain perfusion, suggesting the efficiency of these peptide vectors in delivering doxorubicin to the brain parenchyma. Because the amount of free doxorubicin crossing the BBB is very low, resulting in dpm close to the background of the detection method, the distribution between brain compartments could not be performed for this compound. However, we had previously shown in rats, using a similar method, that about 50% of free doxorubicin was distributed in the parenchyma (Rousselle et al., 2000) and that coupling of doxorubicin with SynB1 led to a 20-fold increase in the amount of doxorubicin transported into brain parenchyma.

We then explored the mechanism by which these dox-peptide complexes cross readily the BBB. The kinetic analysis of [¹⁴C]dox-SynB was determined by using increasing concentrations of the compound ranging from 0.5 to 100 µM. The results of this study demonstrate that vectorized doxorubicin enters the brain via a saturable mechanism that can be described by Michaelis-Menten-type kinetics, exhibiting a relatively high affinity and a low capacity (Table 1). A diffusional component was not necessary to model the data because the amount of vectorized doxorubicin crossing the BBB by passive diffusion over 60 s is not significant. Some other peptides have been found to enter the central nervous system by several saturable mechanisms, including adsorptive-mediated endocytosis (Tamai et al., 1997), receptor-mediated transport (Pardridge et al., 1987), and carrier-mediated transport (Banks et al., 1986). The observed K_m values (4.1-9 µM) measured in this study are comparable to those for substrates reported to be taken up into brain endothelial cells via the adsorptive-mediated endocytosis mechanism, including ebiratide (15.9 µM; Terasaki et al., 1992), E-2078 (4.62 µM; Terasaki et al., 1989), histone (15.2 µM; Pardridge et al., 1989), and cationized bovine serum albumin (0.8 µM; Kumagai et al., 1987). The K_d values for receptor-mediated endocytosis reported for atrial natriuretic factor (0.4 nM; Smith et al., 1988), transferrin (5.6 nM; Pardridge et al., 1987), and insulin (2.3 nM; Frank et al., 1986) are several thousand times smaller than those for adsorptive-mediated endocytosis. Moreover, the fact that the D-SynB3 composed entirely of D-amino acids increased the brain uptake of doxorubucin with the same efficiency as L-SynB3, suggests that the mechanism of transport is nonstereospecific, as would be required for receptor-mediated transport. All these data indicate that the mechanism of transport of dox-SynB is unlikely to be via receptor-mediated transcytosis.

In an attempt to further clarify the saturable mechanism by which SynB vectors transport doxorubicin across the BBB, we examined the brain uptake in the presence of polycationic substances. First, we assessed the effect of poly(L-lysine), a known endocytosis inhibitor. Poly(L-lysine) has been shown to inhibit the binding of E-2078 (Terasaki et al., 1989) and ebiratide (Terasaki et al., 1992). Intracarotid infusions of 5 mg of polycations [poly(L-lysine) and protamine] have also been reported to induce an extravasation of albumin, suggesting an opening of the BBB (Westergren and Johansson, 1993). However in this study, the sucrose space was systematically measured to assess the effect of polycations on the basal permeability of the BBB. The values measured for basal, poly(L-lysine)- (25 µM), and protamine (25 µM)-pretreated mice are well within the range of normal sucrose spaces reported previously (Dagenais et al., 2000). Therefore, in the conditions used in our studies, poly(L-lysine) and protamine have no significant effect on the basal permeability of the BBB during a 60-s exposure. The discrepancy observed between the studies of Westergren and Johansson (1993) and our own might be explained by the higher concentrations of polycations used in the former. Our results showed that the uptake of vectorized doxorubicin was inhibited in a dose-dependent manner by poly(L-lysine), suggesting an endocytosis mechanism. Infusion of polycationic molecules usually results in neutralization of the negative surface charges (Nagy et al., 1983). It is known that the endothelial cell membranes of the BBB have a net negative charge (Vorbrodt, 1989) originating from the sialic acid or heparan sulfate residues on the surface of endothelial cells. The peptides used in this study are positively charged (six positive charges for SynB1 and five positive charges for both L- and D-SynB3), this net positive charge is likely to play a major role in the adsorptive-mediated endocytosis. This is confirmed by the fact that when doxorubicin was coupled to a peptide vector in which positively charged residues have been replaced by neutral amino acids, no brain uptake was seen (data not shown). The inhibitory effect on brain penetration of dox-D-SynB3 observed with polycationic compounds strongly suggests that electrostatic interactions of the peptide vector with the surface of endothelial cells play an important role in the surface binding and subsequent internalization of the vectorized doxorubicin into the brain capillaries. This kind of electrostatic interactions between cationic proteins and negative charges mediate the adsorptive endocytosis (Vorbrodt, 1989). All these results indicate that the vectorized doxorubicin might be transported by an adsorptive-mediated endocytosis system.

For the transcytosis of peptides through the BBB, three steps have been proposed: 1) binding and internalization at the luminal side of endothelial cell membrane, 2) diffusion through the cytoplasm of endothelial cells, and 3) externalization at the abluminal side of endothelial cell membrane (Bar et al., 1983). Our results suggest that adsorptive-mediated endocytosis occurs at least at the luminal side of brain capillaries. The similarity in behavior observed for the three peptide vectors used in this study suggests that the externalization at the abluminal side of endothelial cells may also not be via a receptor-mediated mechanism. However, because the endocytosis inhibitors have only been tested at the luminal side of the endothelial cells, one cannot exclude that a different mechanism may be involved in the externalization step.

Few strategies have been described that result in an improved uptake of doxorubicin into cells. Doxorubicin has been given in combination with P-gp inhibitors (Kusunoki et al., 1998) but if such combinations are effective in vitro, results of studies in patients with solid tumors have been until now somewhat disappointing (Fisher and Sikic, 1995). Arap et al. (1998) have identified peptides that specifically ferry doxorubicin to tumors in nude mice implanted with breast cancer xenographts but to our knowledge, no data regarding the brain uptake have been disclosed. Gulyaev et al. (1999) have observed high brain concentrations of doxorubicin bound to nanoparticles overcoated with surfactant polysorbate 80 but this effect may be related to the toxicity of the carrier against the BBB (Olivier et al., 1999).

Our studies indicate that coupling doxorubicin with SynB vectors enhances its brain uptake. The vectorized doxorubicin bypasses P-gp at the BBB and is most likely transported via adsorptive-mediated transcytosis. Our results suggest the possibility of using this system for the treatment of brain cancer and other central nervous system diseases. Toward this goal, future studies are in progress to explore the biological activity of vectorized drugs in animal models.