Dinuclear Platinum Complexes as potential anticancer drugs : insights
in the intracellular distribution
Kalayda, G.V.
Citation
Kalayda, G. V. (2006, February 1). Dinuclear Platinum Complexes as potential anticancer
drugs : insights in the intracellular distribution. Retrieved from
https://hdl.handle.net/1887/4283
Version:
Corrected Publisher’s Version
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Licence agreement concerning inclusion of doctoral thesis in the
Institutional Repository of the University of Leiden
Downloaded from:
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Chapt
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Abstract
6.1. Introduction
One of the general problems in chemotherapy is the difficulty to reach a therapeutically active concentration of a drug at the site of action.1 Increasing the dose is not the best option, as it may lead to undesirable side effects, for example, severe toxicity.2 Targeting drugs to the therapeutic sites may provide a good solution for the problem. As mentioned in Chapter 1, drug targeting has a number of advantages. Firstly, it increases drug concentration at the active site enhancing the therapeutic effect. At the same time, specific drug delivery decreases the concentration of a drug at non-therapeutic sites, thereby reducing toxicity. However, drug targeting strategy comprises some challenges. Coupling of the drug to the carrier should not result in a significant decrease of the biological activity the drug, and should not impair the targeting ability of the carrier. Furthermore, drug release at the target should be achieved, so that the drug would perform its therapeutic action.
The widely used covalent coupling of therapeutic agents to targeting molecules has several drawbacks. First of all, chemically reactive groups for covalent binding might be not available. Secondly, covalent linkage might irreversibly inhibit the activity of the coupled drug. Furthermore, the cleavage of the drug at the site of action might be difficult to attain and to control.
Coupling of drugs to carrier molecules based on coordination chemistry presents a completely different approach in the design of targeting conjugates. This strategy has interesting advantages over the covalent coupling. It allows coupling of the molecules, which lack chemically reactive groups for covalent binding. If a drug is conjugated to a carrier through a metal center, it will be released in its active form, i.e. targeting will not influence the bioactivity of the drug. Furthermore, the rate of drug release from the metal-based conjugate can quite easily be controlled. As the rate of ligand exchange in metal complexes is dependent on the metal and its coordination surrounding, coupling of a therapeutic agent to a carrier can be designed to be reversible, which will provide the necessary release of the drug at the site of action. The rate of ligand exchange in platinum complexes is relatively slow,3 which would enable the control of drug binding to platinum and its subsequent dissociation from platinum species.
The Universal Linkage System (ULSTM) developed at Kreatech Biotechnology B.V. is a strategy for labeling of nucleic acids and proteins based on cisplatin derivatives (so called ULS reagents).4-7 As shown in Figure 6.1, a standard ULS reagent contains a reporter molecule and a leaving group (usually chloride).
H2N
Pt
NH2
leaving group reporter
A leaving group can be substituted by guanine and adenine of nucleic acids,4 and by cysteine, methionine and histidine amino acid residues of protein molecules.8 ULSTM has been successfully used for labeling and detection of nucleic acids for various applications in molecular and cell biology.9-11 Since ULS reagents readily bind to the afore mentioned amino acid residues, they appear very suitable for coupling of drugs to carrier proteins.
This chapter presents a strategy for coupling small, organic drug molecules to targeting proteins using dinuclear ULS-based complexes with hydrophilic bridging ligands. It is focused on the design of a drug-ULS conjugate schematically depicted in Figure 6.2, a precursor for the synthesis of the platinum-based drug-carrier conjugates.
N H2 Pt H2 N drug N N Pt X N H2 H2 N
N N = diam ine-based linker X = easy leaving group (e.g. Cl) to be substituted by a carrier protein
Figure 6.2. Schematic representation of the desired dinuclear drug-ULS conjugate.
ptx-ULS conjugate 4 (Figure 6.3), a precursor for targeting pentoxyfilline to hepatic stellate cells, and investigation of ptx release from this complex.
N O O N N O H2N Pt NH2 Cl Cl H2N O NH2 1 2 3 (pentoxyfilline, ptx) N H2 Pt H2 N HN2 PTX O H2 N Pt N H2 H2 N Cl 3+ 4
Figure 6.3. Schematic representation of the desired dinuclear drug-ULS conjugate and the starting compounds used as building blocks in its synthesis.
6.2. Experimental section 6.2.1. Instruments
1H and 195Pt NMR spectra were recorded on a Bruker DPX 300 MHz spectrometer with
a 5 mm multi-nucleus probe. The temperature was kept constant by a variable temperature unit. 1H and 195Pt chemical shifts were referenced to TSP and Na2PtCl6 (δ = 0 ppm),
respectively. 195Pt NMR spectra were recorded in undeuterated solvents. C, H and N analyses were performed by microanalytical laboratory of Leiden Institute of Chemistry, Leiden University, the Netherlands.
The mass spectroscopic measurements were performed on a Finnigan Aqa mass spectrometer equipped with an electrospray interface ionization source. Sample solutions were introduced in the ESI source using a HPLC autosampler and an acetonitrile/water (50/50) mixture as an eluent running at 0.2 ml/min.
LC ESI-MS experiments were carried out on a Alltima 3µ C18 reversed phase column (150× 4.6 mm) with a flow rate of 1 ml/min, sample load of 10-50 µl (1 mg/ml) and the following gradient conditions: 96% B for 15 min, then from 96% B to 80% B in 5 min, and subsequently from 80% B to 60% B in 5 min (eluent A: 30% 100mM TEAA pH 5, 70% acetonitrile/ eluent B: 100mM TEAA pH 5; where TEAA is triethylammonium acetate). After the column separation, the flow is split. One part flows through the UV-detector (254 nm); the other part (0.2 ml/min) is directed to the Finnigan Aqa mass detector operating in electrospray mode.
5.9 min, and subsequently from 80% B to 60% B in 5.9 min (eluent A: 30% 100mM TEAA pH 5, 70% acetonitrile/ eluent B: 100mM TEAA pH 5). Fractions were collected, analyzed by mass spectroscopy, and the desired fraction was lyophilized.
6.2.2. Synthesis
The synthetic schemes for the preparation of the desired compounds are presented in Figure 6.4 and Figure 6.5.
Materials
K2PtCl4was obtained from Johnson&Matthey. Silver nitrate, triethylamine and
di-tert-butyl dicarbonate were ordered from Fisher Scientific Nederland. 2-(2-aminoethoxy)ethanol, methanesulfonyl choride and triphenylphosphine were purchased from Aldrich. Sodium azide, ethylenediamine (en), potassium iodide and potassium chloride were obtained from Merck. Pentoxyfilline (Sigma) was obtained from Dr. R. J. Kok (Groningen University Center for Pharmacy).
Dichloro(ethylenediamine)platinum(II) (1)
Potassium iodide (2g, 12 mmol) was added to a solution of 1 g (2 mmol) of K2PtCl4 in
50 ml of water. The resulting dark solution of K2PtI4 was treated with 0.16 ml (2.4 mmol) of
ethylenediamine and allowed to stand at room temperature for several hours. Then, the dark yellow precipitate of Pt(en)I2 was collected by filtration, washed with water, ice-cold ethanol
and ether, and dried in air. Yield: 1 g (99%).
Silver nitrate (0.66 g, 3.9 mmol) in 5 ml of water was then added to the suspension of Pt(en)I2 (1 g, 2 mmol) in 25 ml of water upon stirring. The mixture was stirred in the dark
overnight at room temperature. Then, the white precipitate of AgI was filtered off, and 0.33 g (4.4 mmol) of KCl was added to the filtrate. The mixture was stored overnight in the dark, and the resulting yellow precipitate was collected by filtration, washed with ice-cold water, ethanol and ether, dried in air. Yield: 0.52 g (80%). Anal. Calcd for C2H8N2Cl2Pt: C, 7.37; H,
2.47; N, 8.59. Found: C, 7.58; H, 2.80; N, 8.50. 195Pt NMR (H2O): δ -2330.
2-(2-(tert-butyloxycarbonylamino)ethoxy)ethanol (2a)
A solution of 2.62 g (12 mmol) of di-tert-butyl dicarbonate in 10 ml of ethanol was gently added to a stirred solution of 1.05 g (10 mmol) of 2-(2-aminoethoxy)ethanol in 10 ml of ethanol. After stirring for 4 h at room temperature, the solvent was removed under reduced pressure. The residue was extracted with 25 ml of ethylacetate, the organic phase was dried (Na2SO4), filtered and evaporated to get the product. Yield: 1.95 g (95%). 1H NMR (CDCl3):
δ 4.88 (s, 1H, NH), 3.74 (t, 2H, H1), 3.56 (m, 4H, H2+ H3), 3.34 (m, 2H, H4), 1.45 (s, 9H,
Boc).
1-(2-(tert-butyloxycarbonylamino)ethoxy)-2-sulfonylethane (2b)
room temperature. Then the solution was successively washed with 1 M HCl (50 ml), water (50 ml), 10% aqueous solution of Na2CO3(50 ml) and brine (50 ml). The organic phase was
dried (Na2SO4), filtered and evaporated to get the product. Yield: 1.62 g (60%). 1H NMR
(D2O):δ 4.35 (t, 2H, H1), 3.73 (t, 2H, H2), 3.56 (t, 2H, H3), 3.33 (m, 2H, H4).
1-(2-(tert-butyloxycarbonylamino)ethoxy)-2-azidoethane (2c)
Sodium azide (1.49 g, 22.9 mmol) was added to a stirred solution of 2b (1.62 g, 5.72 mmol) in 20 ml of anhydrous dimethylformamide. The mixture was heated at 60-70 °C overnight. After cooling, the solution was poured into water and extracted with ethyl acetate (5×30 ml). Organic phases were collected, dried (Na2SO4), filtered and evaporated to obtain
the product. Yield: 0.6 g (46%). 1H NMR (CDCl3):δ 3.65 (t, 2H, H2), 3.55 (t, 2H, H3), 3.36
(m, 4H, H1+ H4).
1-(2-(tert-butyloxycarbonylamino)ethoxy)-2-aminoethane (NONBoc, 2d)
A solution of 0.82 g (3.1 mmol) of triphenylphosphine in 10 ml of tetrahydrofuran was added to a solution of 0.6 g (2.6 mmol) of 2c in 20 ml of tetrahydrofuran. The resulting solution was stirred for 2 h at room temperature. Subsequently, 200 ml of water was added, and the mixture was stirred overnight at room temperature. Then the mixture was evaporated to 75 ml and filtered. The filtrate was washed with 25 ml of dichloromethane and evaporated to get the product. Yield: 0.5 g (79%). 1H NMR (CDCl3):δ 3.50 (m, 4H, H2+ H3), 3.33 (m,
2H, H4), 2.86 (t, 2H, H1). ESI-MS: m/z 205.2 (M+H).
[Pt(en)Cl(dmf)](NO3) (1a)
A solution of 38.6 mg (0.23 mmol) of AgNO3 in 1 ml of dimethylformamide (DMF)
was added portionwise over 1.5 h to a stirred solution of 78.2 mg (0.24 mmol) of 1 in 1.6 ml of DMF at room temperature in the dark. The mixture was stirred for 5 h in the dark, and the AgCl precipitate was then filtered off. The resulting pale-yellow solution of [Pt(en)Cl(dmf)](NO3) was used as a starting material for the preparation of 1b and 1c as
described below.
[Pt(en)Cl(ptx)](NO3) (1b)
A solution of 1a obtained as above was added to 43.8 mg (0.16 mmol) of ptx in the dark. The resulting solution was stirred overnight in the dark at 80 °C. Then the solvent was removed in vacuo. Excess of water was added to the residue, and a yellow precipitate of Pt(en)Cl2 was filtered off. The remaining filtrate of 1b was lyophilized. Yield: 100 mg. 195Pt NMR (H
2O): δ -2474.
[Pt(en)Cl(NONBoc)](NO3) (1c)
A solution of 1a obtained as above was added to 40 mg (0.2 mmol) of NONBoc (2d) in the dark. The resulting solution was stirred overnight in the dark at room temperature. Then DMF was removed in vacuo. Excess of water was added to the residue, and a yellow precipitate of Pt(en)Cl2 was filtered off. The remaining filtrate of 1c was lyophilized. Yield:
[Pt(en)(ptx)(NONBoc)](NO3) (1f), route I
A solution of 14.5 mg (0.085 mmol) of AgNO3 in 0.6 ml of DMF was added
portionwise over 1.5 h to a stirred solution of 56.8 mg (0.09 mmol) of 1b in 2.4 ml of DMF at room temperature in the dark. The mixture was stirred for 4.5 h in the dark, and the AgCl precipitate was then filtered off. The filtrate 1d was added to the solution of 19 mg (0.09 mmol) of 2d in 0.4 ml of DMF. The resulting solution was divided in two. One part was allowed to stir at room temperature in the dark for 72 h, and the other part was stirred in the dark at 50 °C for 72 h. Both reactions were monitored by 195Pt NMR spectroscopy.
[Pt(en)(ptx)(NONBoc)](NO3) (1f), route II
A solution of 19.8 mg (0.117 mmol) of AgNO3 in 1 ml of DMF was added portionwise
over 1.5 h to a stirred solution of 68.6 mg (0.123 mmol) of 1c in 1.6 ml of DMF at room temperature in the dark. The mixture was stirred for 5 h in the dark, and the AgCl precipitate was then filtered off. The filtrate 1e was added to 27.4 mg (0.098 mmol) of ptx. The resulting solution was stirred in the dark at 70 °C. Then it was filtered off the black precipitate, and evaporated. The residue was dissolved in water, filtered, and the filtrate of 1f was lyophilized. Yield (crude product): 71 mg. 195Pt NMR (DMF): δ -2700. ESI-MS: m/z 737.2 (M–2(NO3)–H), 459.28 (M–2(NO3)–ptx–H).
[{Pt(en)(ptx)}(µ-NON){Pt(en)X}](CH3COO)3, X = Cl (4) or CH3COO (4a)
Complex 1f (81.4 mg, 0.094 mmol) was dissolved in 4 ml of 0.1 M hydrochloric acid, and the resulting solution was heated overnight in the dark at 50 °C. After that, pH of the solution was adjusted to 8 with 2 M NaOH. Then 68 mg (0.4 mmol) of silver nitrate was added, in order to remove chloride ions from the solution. The precipitate of AgCl was filtered off, and subsequently a solution of 1a obtained as described above was added to the filtrate. The resulting solution was stirred at 50 °C for 24 h in the dark. The solvent was then removed in vacuo. The residue was dissolved in water, filtered and purified by high performance liquid chromatography as described above. The purified product was lyophilized and characterized by 1H and 195Pt NMR and LC ESI-MS. Yield: 5 mg. 1H NMR (D2O):δ 8.61 (s, 1H, ptx), 4.50 (s, 3H, ptx), 4.09 (s, 3H, ptx), 4.01 (t, 2H, NON), 3.72 (t, 2H,
NON), 3.64 (m, 4H, NON), 2.81 (m, 2H, en), 2.78 (m, 2H, en), 2.65 (m, 4H, ptx), 1.93 (s, CH3COO), 1.62 (m, 4H, ptx). 195Pt NMR (D2O):δ -2400 (4a), -2632 (4), -2700. LC ESI-MS:
retention time for 4 9.68 min (m/z 985.14 (M(4)3+-2H+CH3COO), 647.04 (M(4)3+-3H-ptx);
retention time for 4a 12.09 min (m/z 1009.08 (M(4a)3+-2H +CH3COO), 669.93 (M(4a)3+
-3H-ptx), where M(4)3+ is a mass of the cation [Pt(en)(ptx)(µ-NON)Pt(en)Cl] 3+ and M(4a)3+ is a mass of the cation [Pt(en)(ptx)(µ-NON)Pt(en)(OCOCH3)] 3+.
6.2.3. Investigation of ptx release from the dinuclear ptx-ULS conjugate (the mixture of 4 and 4a obtained as above)
were incubated at 37 °C for 16 days. From both solutions, an aliquot was taken every day for LC ESI-MS analysis as described above. The decomposition reactions were quantified based on the relative integration values of the UV peaks of free ptx in the chromatograms.
6.3. Results and discussion
6.3.1. Synthesis of the dinuclear drug conjugate
The synthetic scheme for the preparation of the desired dinuclear ptx-ULS conjugate 4 is presented in Figure 6.4. N H2 Pt H2 N Cl Cl N H2 Pt H2 N dmf Cl N H2 Pt H2 N ptx Cl N H2 Pt H2 N Cl N H2 O NHBoc N H2 Pt H2 N dmf N H2 O NHBoc N H2 Pt H2 N ptx dmf N H2 Pt H2 N ptx N H2 O NHBoc 1 1a 1b 1c 1d 1e 1f N H2 Pt H2 N ptx N H2 O NHBoc 1f H2 N Pt N H2 N H2 ptx O N H2 Pt H2 N N H2 Cl N H2 Pt H2 N ptx N H2 O NH2 1g 4 AgNO3 DMF AgNO3, DMF AgNO3, DMF - AgCl - AgCl - AgCl ptx ptx 2. 1a, 50°C (2d) H2N O NHBoc 2d 1. AgNO3; - AgCl HCl 50 °C 80 °C 70 °C + + + 2+ 2+ 2+ 2+ 2+ 3+
Figure 6.4. Synthetic scheme for the preparation of the dinuclear conjugate 4
this reason, the mononuclear precursor 1f was prepared. Two synthetic routes (I and II) were considered. Route I includes ptx binding to the ULS reagent 1 followed by the reaction with the monoprotected diamine ligand 2d, and route II implies the reverse order of binding the ligands to ULS. Prior to the reaction with platinum, one of the two amino-groups of the bridging ligand 2 was protected with the Boc-group to ensure that only the mononuclear product will be formed upon platination. The synthesis of the monoprotected diamine 2d was performed using the modification of the method described by Benoist et al.18 (Figure 6.5).
H2N O OH Boc-HN O OH Boc-HN O OSO2Me Boc-HN O N3 Boc-HN O NH2 Boc2O MeSO2Cl NaN3 1. Ph3P 2. H2O 2d 60-70 °C 2a 2b 2c 1 2 3 4 1 1 1 2 2 2 3 3 4 4 4 3
Figure 6.5. Synthetic scheme for the preparation of the monoprotected diamine 2d
In the multi-step synthesis of complex 1f, one of the chloride ligands of the ULS reagent 1 was first replaced with a solvent molecule (dimethylformamide, DMF). The resulting species 1a is much more reactive, because DMF is an easy leaving group compared to chloride. Complex 1a was then directly reacted either with pentoxyfilline (route I) or with the monoprotected diamine 2d (route II).
As had been expected, PTX was found to readily interact with the activated ULS reagent (route I) resulting in the formation of 1b in a good yield. The structure of 1b was confirmed by 195Pt NMR spectroscopy19 (Table 6.1). Reaction of 1a with 2d (route II) led to the formation of complex 1c, as was clear from the 195Pt NMR spectrum of the product11 (Table 6.1). After both reaction mixtures were redissolved in water, excess of the ULS complex was easily removed by filtration yielding purified compounds 1b and 1c, respectively. Then, complex 1b was activated to obtain reactive species 1d and further reacted with the monoprotected diamine 2d (route I). However, monitoring the reaction with
195
Pt NMR spectroscopy showed no interaction between the platinum complex and the amine. Only the peak at -2195 ppm, which corresponds to [N3O] coordination surrounding of
may appear controversial, as two molecules of structurally similar guanine are known to bind to platinum in cis-position in biological systems.17
In contrast, route II was found to be successful, as reactive species 1e (the activated form of 1c) reacts with ptx to form the desired mononuclear precursor 1f. The structure of 1f was confirmed by 195Pt NMR spectroscopy19 (Table 6.1) and mass spectrometry. This complex was used in the following synthetic steps as obtained, in order to avoid the significant decrease in the yield upon purification.
Thus, only a certain sequence of binding the ligands to a ULS reagent provides the desired mononuclear platinum-drug conjugate. Complex 1f can only be obtained if binding of the aliphatic amine to ULS is followed by the reaction with ptx, and not the other way around. This shows that the design of drug-carrier conjugates based on coordination chemistry is not straightforward and requires detailed investigation.
Table 6.1. 195Pt NMR data for the mononuclear platinum complexes 1, 1b, 1c, 1f and the dinuclear conjugates 4 and 4a
complex δ, ppm platinum coordination surrounding
1 -2330 [N2Cl2]21 1b -2474 [N3Cl]19 1c -2645 [N3Cl]11 1f -2700 [N4]19 4 -2630 -2700 [N3Cl]11 [N4]19 4a -2400 -2700 [N3O]22 [N4]19
Complex 4 was purified by HPLC using triethylammonium acetate (TEEA) buffer as an eluent. The fractions were collected and their mass spectra were measured. The desired fraction was subsequently lyophilized to get the product. Unfortunately, the yield of 4 after purification was low. Besides, HPLC purified dinuclear ptx-ULS conjugate 4 represents a mixture of the desired complex 4 (approx. 75% based on the relative integration values of the UV peaks in the HPLC chromatogram) and complex 4a (approx. 25%), which features leaving acetate group instead of chloride (Figure 6.6).
N H2 Pt H2 N HN2 ptx O H2 N Pt N H2 H2 N Cl 4 (CH3COO)3 N H2 Pt H2 N HN2 ptx O H2 N Pt N H2 H2 N CH3COO 4a (CH3COO)3
Figure 6.6. Schematic representation of the complexes comprising the product (complexes 4 and 4a), and the results of LC ESI-MS analysis of the product mixture.
D:\MS_data\...\Ak2703-1-2_LCMS_C218 04/08/04 02:28:15 PM in ACN/H2O
Ak2703-1-2_LCMS_C218 #929-967RT:11.95-12.44AV:39SB:13 0.05-0.20NL:1.39E3
F:{0,0} + c ESI sid=30.00 Full ms [ 300.00-1100.00]
300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 m/z 0 200 400 600 800 1000 1200 In te n s it y 1009.08 1010.15 1008.01 1011.13 731.12 669.93 1012.20 1007.02 515.10 672.26 733.12 874.93 1006.38 1014.07 669.06 498.84 530.90 597.07 707.84 753.21 812.07 948.13 1090.11 375.24392.60441.61 902.85 305.40
Ak2703-1-2_LCMS_C218 #731-776RT:9.41-9.98AV:46SB:13 0.05-0.20NL:2.16E3
F:{0,0} + c ESI sid=30.00 Full ms [ 300.00-1100.00]
Most likely, the chloride ligand is replaced by acetate on the column. The product was characterized by 1H and 195Pt NMR spectroscopy (Table 6.1) and LC ESI-MS (Figure 6.6). It was not purified further, as the acetate complex 4a is equally suitable for coupling to carrier proteins as the chloride complex 4. Both acetate and chloride can be substituted by nitrogen and sulfur donor atoms of a carrier. Thus, the obtained mixture of the desired dinuclear ptx-ULS conjugates, which will be further referred to as product 4, can be used for coupling to a targeting protein.
6.3.2. Drug release from the dinuclear conjugate
The stability of the dinuclear conjugate 4 with respect to pentoxyfilline release was investigated at 37 °C at pH 7 and pH 5 by LC ESI-MS analysis. Decomposition of product 4 was quantified based on the relative integration values of the UV peaks of free ptx in the chromatograms. Figure 6.7 shows ptx release from product 4 at different pH at 37 °C. For comparison, the data on pentoxyfilline release from the mononuclear drug-ULS conjugate 1b at the same conditions investigated at Kreatech Biotechnology B.V.23 have been included.
Figure 6.7. Pentoxyfilline release from the dinuclear ptx-ULS conjugate (product 4) and the mononuclear ptx-ULS conjugate (complex 1b).
As is clear from Figure 6.7, drug release from the dinuclear conjugate product 4 is faster at pH 7 than at pH 5. The same trend was observed in the case of the mononuclear complex. However, the mononuclear conjugate is much more stable. For example, after
4 days of incubation at 37 °C the mononuclear complex 1b loses approximately 1% of pentoxyfilline, while drug release from the dinuclear conjugate product 4 reaches 5%. Furthermore, the difference in the drug release rate at pH 5 and pH 7 is much larger in the dinuclear case than in the mononuclear case.
It is not yet known what rate of drug release is favorable for the biological activity of the targeted drug, and for that reason, it is now not possible to say whether the dinuclear complex has an advantage over the mononuclear one. It is also not yet clear which factors control the stability of ptx conjugates. Controlled drug release is very important in drug targeting, and it deserves much attention in further studies.
6.4. Conclusions
Dinuclear cis-configured platinum complexes are suitable for coupling of small organic drugs to targeting proteins. However, the preparation of dinuclear drug-platinum conjugates for further binding to a carrier is not straightforward and includes multi-step synthesis and thorough purification. Besides, the overall yield of the dinuclear complex is low. Therefore, it is doubtful that dinuclear platinum complexes will find a broad application in drug targeting. Drug release from the dinuclear conjugate is faster than from the mononuclear complex. However, it is not clear whether faster drug dissociation from platinum species presents an advantage. The factors, which influence drug release from platinum-based conjugates, are not yet well elucidated and require further detailed investigation.
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