SEMIFLUORINATED SYMMETRICAL DIETHERS FOR INTRAOCULAR USE: SYNTHESIS, CHARACTERIZATION, AND IN VITRO BIOCOMPATIBILITY

Abstract

Semifluorinated symmetrical diethers were synthesized using the William ether synthesis. These diethers should have similar properties to perfluorocarbons as are chemical inertness and high oxygen solubility, but in contrast a considerably lower density. With their lower density the damaging of the choroidal tissue of the eye observed with perfluorocarbons should be avoided. The synthesized diethers are inert compounds being stable against nucleophiles, oxidiziers and strong bases. Their density is in the range of 1.1–1.2 g/cm³. Besides the physical and chemical tests we conducted several in vitro biocompatibility tests. The tests comprised induction of hemolysis, the generation of C3a complement, the influence on the production of interleukin1β, the influence on cell proliferation of a Raji and a Hela cell line (³H-Thymidine uptake) and finally the direct cytotoxic effect on these cell lines. All tested symmetrical diethers were positive in one or more tests and can be expected to be incompatible in vivo. Especially the “short” semifluorinated diethers [(CF₃CH₂O)₂(CH₂)_3–6] showed a nearly total inhibition of cell proliferation or interleukin1β release. Further variation of the compounds will be necessary to generate better biocompatible derivates.

INTRODUCTION

More than thirty years ago, Clark and Gollan [[1]] showed that mammals were able to survive while breathing organic liquids. Since then, various perfluorocarbons have been developed and used in experimental and clinical applications—for example, blood substitute [2-3], contrast agent [4-5], liquid ventilation [6-7] or retina fixation [[8]]. In the case of retina fixation, the high density of perfluorocarbons is suggested to be responsible for damaging the choroidal tissue [9-10]. Therefore symmetrical diethers were synthesized, having such similar properties to perfluorocarbons as chemical inertness and high oxygen solubility but considerably lower density [[11]].

For the present work we synthesized semifluorinated symmetrical unbran-ched diethers by the William ether synthesis [[12]]. The synthesized diethers are a homologous series of trifluorethanol of the type (CF₃CH₂O)₂(CH₂)_x. These semifluorinated diethers possess an oxygen solubility of 32–33 vol.-% O₂ at 37°C [[11]]. The data of density, stability, and interfacial and surface tension will be presented. In addition, we determined the in vitro biocompatibility profile of these diethers. For direct cytotoxicity, the degree of hemolysis and the influence of the diethers on the survivial of human cells (a Raji and a Hela cell line) is determined. The influence on cell proliferation of these cell lines is examined by ³H-thymidine uptake. Furthermore, the ability of the diethers to induce the generation of C3a complement is determined. C3a is a cleavage product of C3, and C3 is activated not only by the classical pathway but also by the alternative pathway. Finally, we examined the influence of the semifluorinated diethers on the generation of interleukin-1β. Interleukin-1β is released by activated monocytes, neutrophiles, and several lymphocytes and plays a key role in the defense of infectious diseases, tissues damage, and inflammation.

MATERIALS AND METHODS

Peripheral leukocytes, serum, and erythrocytes were obtained from healthy volunteer donors.

Cell Lines

Hela cells are human cervix carcinoma cells [[13]] growing as a monolayer. The cells were purchased from the German Collection of Microorganisms and Cell Cultures (Braunschweig, Germany). They were cultured with Dulbecco's modified Eagle medium (Gibco, Eggenstein, Germany), containing 2 mM glutamin (Gibco), supplemented with 10% heat-inactivated fetal calf serum (Roth, Karlsruhe). Raji cells are lymphoblast-like cells from a human Burkitt lymphoma [[14]] growing freely in the fluid phase. The cells were purchased from the American Type Culture Collection. They were grown in RPMI 1640 medium (Gibco) supplemented with 20% heat-inactivated fetal calf serum and 2 mML-glutamin.

Test Substances

Ringer's lactate (Baxter, Unterschleissheim, Germany) was used. A synthesis of semifluorinated symmetrical diethers: trifluoroethanol (Solvay, Germany), CF₃CH₂OH, (0.6 mol), was added dropwise to metallic sodium (0.6 mol) at such a rate that the reaction temperature did not exceed 60°C. (An excess of 20% CF₃CH₂OH was used as solvent.) The reaction mixture was refluxed for 2 h, then a,w-dibromoalkane (0.3 mol; Fluka, Germany) was added dropwise. After refluxing the solution for 20 h under nitrogen (N₂), 200 mL of a 5% aqueous solution of NaOH was added. The product was extracted with diethyl ether (200 mL) and dried (anhydrous Na₂SO₄). After removing the solvent, the liquid residue was fractionally distilled. The synthesized diethers are listed in Table 1 (see Results).

Table 1. Synthesized Semifluorinated Diethers

Compound	M [g/mol]	b.p. [°C/mm Hg]	φ [g/cm^3]
(CF3CH2O)2(CH2)3	240	152/760	1.23
(CF3CH2O)2(CH2)4	254	76/16	1.21
(CF3CH2O)2(CH2)5	268	92/15	1.18
(CF3CH2O)2(CH2)6	282	107/18	1.16
(CF3CH2O)2(CH2)71^a	296	122/15	1.13
(CF3CH2O)2(CH2)81^a	310	137/15	1.11
(CF3CH2O)2(CH2)91^a	324	151/15	1.08
(CF3CH2O)2(CH2)101^a	338	166/15	1.06

b.p.: boiling point, φ: density.

^aNew.

Stability of Semifluorinated Diethers

The stability of semifluorinated diethers was tested in the following way:

1. Resistance toward strong bases; contact with 8 N potassium hydroxide solution for 16 h at 120°C

2. Resistance toward strong oxidizers; contact with potassium permanganate (2 N) in acidic medium; (1 N H₂SO₄) for 8 h at 70°C

3. Resistance against nucleophiles; contact with hexamethylenediamine (25%) in decane at 120°C

Reaction mixtures were monitored by gas chromatography methods (column I) against perfluorodecalin.

Interfacial and Surface Tension of Semifluorinated Diethers Against Water

Surface tension and interfacial tension against H₂O were measured on a K 8 tensiometer (Krüss) using the ring method [[15]]. Results were corrected by the method of Zuidema and Waters [[16]].

Gas Chromatography

Gas chromatography was carried out on a Gira CAP 12 gas chromatograph equipped with a thermal conductivity detector, using columns I and II for analytical separations. Column I: copper column (5 m × 4 mm i.d.) packed with 10% SE 30 on 80–100 mesh Chromosorb P. Column II: glass column (1.7 m × 3 mm i.d.) packed with 40–60 mesh 5-Å molecular sieve.

Sterility Control

For all test substances, controls of sterility were made every time before use in the tests. The substances were plated onto blood/agar plates and incubated at 37°C, 5 vol.-% CO₂, in humid atmosphere. After 24 and 48 h the plates were controlled for bacterial growth. None of the substances used in the tests showed bacterial growth. All probes negative in sterility tests were passed through 0.4-μm filters prior to use in compatibility tests.

Hemolysis Assay

A 20% (w/v) erythrocyte suspension was prepared with Ringer's lactate. To determine the hemolytic activity of the test substances, 50 μL of the erythrocyte suspension was incubated together with 100 μL of the test substance plus 100 μL of serum or isotonic saline. Erythrocyte suspension with isotonic saline was used as negative control; erythrocyte suspension with distilled water was used as positive control. The samples were incubated for 1 h at 37°C in a water bath, then the samples were centrifuged for 2 min at 2.000g (Heraeus Instruments, Stuttgart, Germany) and the optical density of the supernatants was measured spectrophotometrically (SLT Labinstruments, Crailsheim, Germany). A test substance induced hemolysis when the following quotient was greater than 2:where OD_{t + R.L./s + e} is the optical density of the supernatant consisting of test substance (t) plus Ringer's lactate or serum plus erythrocytes (e); OD_{t + R.L./s} is the optical density of the supernatant consisting of test substance plus Ringer's lactate or serum; and OD_{R.L./s + e} is the optical density of the supernatant consisting of Ringer's lactate or serum plus erythrocytes.

Complement Activation Assay (C3a)

For the complement activation assay, fresh sterile serum was used each time. It was prepared as follows. Blood was obtained by sterile venepuncture. The blood was allowed to clot over 5 min at 37°C. Then it was centrifuged over 10 min at 4°C and 800g (Heraeus Instruments, Stuttgart, Germany). For the assay, 100 μL of serum were incubated together with 100 μL of test substance or control. Before reaction, all samples were stored on ice. For the negative control, serum was incubated together with Ringer's lactate; for the positive control, Zymosan (Sigma, Deisenhofen) in Ringer's lactate at a final concentration of 5 mg/mL was used. The samples were incubated at 37°C over 30 min. The reaction was stopped using EDTA (Gibco, Eggenstein, Germany) at a final concentration of 50 mmol/L. Then the samples were centrifuged over 2 min at 800g (Heraeus Instruments, Stuttgart, Germany). For the determination of the C3a desarg. concentrations of the supernatants, we used an ELISA (Progen, Heidelberg, Germany).

Cytokine Assay

Mononuclear leukocytes were prepared with gradient separation (Lymphoflot separation medium, Biotest, Dreieich, Germany) using human blood (buffy coat). The mononuclear leukocytes were used with a final concentration of 1 × 10⁶ cells/mL and maintained with 5 vol.-% fetal calf serum (Roth, Germany) and RPMI (1640, Gibco, Eggenstein, Germany). One hundred microliters of the cell suspension was incubated for 4 h at 37°C, 5 vol.-% CO₂, humid atmosphere, together with 50 μL of the test substance or the controls in flat-bottom microtiter plates (96 wells, Falcon, Becton Dickinson, Heidelberg, Germany). Then lipopolysaccharide (Salmonella typhimurium, Difco, Augsburg, Germany) was added to a final concentration of 10 ng/mL and the cell suspension was incubated for another 20 h. Thereafter, the interleukin-1β concentrations of the supernatants were measured using the ELISA technique (IL1β Quantikine, DPC, Bad Nauheim, Germany; IL6, Pharmingen, Hamburg, Germany). The degree of suppression or induction of interleukin-1β (IL1β) production was determined by the following quotients:with [IL1β]_{MNL+test compounds+LPS} is the concentration of IL1β after incubation of mononuclear leukocytes (MNL) with different test compounds plus lipopolysaccharide (LPS) and [IL1β]_MNL+LPS is the concentration of IL1β after incubation of mononuclear leukocytes (MNL) with LPS.

Cell Proliferation Assay

For this assay, 96-well culture plates (Falcon, Becton Dickinson, Heidelberg, Germany) were used. Each well contained 1 × 10⁵ cells (Hela) or 5 × 10⁵ cells (Raji) in a volume of 100 μL. To each well were added 50 μL of a test substance or the control (culture medium). The test substances and the controls were placed symmetrically and alternately to avoid position effects on the microtiter plate. The culture plates were incubated for 24 h at 37°C, 5 vol.-% CO₂, humid atmosphere. Then 20 μL of ³H-thymidine (37 MBq/mL, Amersham, Braunschweig, Germany) were added to each well. After another 24 h of incubation at 37°C, the culture plates were harvested onto glass-fiber filters using an automated cell harvester. Because Hela cells are adherent cells, they were trypsinized before harvesting (trypsin from Biochrom, Berlin, Germany). Finally the cell proliferation was determined by scintillation counting (Betaplate, LKB Wallac/ Pharmacia, Finland). The relative ³H-thymidine incorporation was calculated as follows:where tc is the test compound and cm is the culture medium.

Toxicity Assay (Light Microscopy)

For the toxicity assay, Raji or Hela cells were used at a concentration of 2 × 10⁶ cells /mL in their specific medium (see cell cultures). For each sample, 100 μL of the cell suspension was incubated with 100 μL of each test substance in 4-mL tubes (Greiner, Solingen, Germany). The samples were incubated for 24 h at 37°C and 5 vol.-% CO₂, humid atmosphere. Thereafter, 100 μL of the suspension and 100 μL of Trypanblue (concentration 0.4 vol.-%, Sigma, Deisenhofen, Germany) were incubated over 4 min. Then, 100 cells were counted by light microscopy (Zeiss, Oberkochem, Germany) and the fraction of dead cells was determined. Each sample was counted twice.

RESULTS

Part I: Physical and Chemical Properties of the Semifluorinated Diethers

Semifluorinated alcoholate anions were prepared by the reaction of metallic sodium at 60°C using excess trifluoroethanol as the solvent:Semifluorinated diethers were prepared by the William ether synthesis using semifluorinated alcoholate anions and different alkyl dihalides: where X = 3–10. The diethers synthesized in this work are listed in Table 1. All products are clear colorless liquids.

Density

As depicted in Table 1, density of the semifluorinated diethers is in the range of 1.1–1.2 g/cm³. Density decreases with increasing hydrocarbon spacer.

Stability of Semifluorinated Diethers

The stability of semifluorinated diethers was tested against strong bases (8 N potassium hydroxide), strong oxidizers (2 N potassium permanganate), and nucleophiles (hexamethylendiamine). In no case was any reaction observed.

Interfacial Tension

While interfacial tension against water decreases with increasing CH₂ spacer length (Fig. 1), surface tension increases with increasing spacer length (Fig. 2).

Figure 1. Interfacial tension against H₂O; T = 21°C.

Figure 2. Surface tension; T = 25°C.

Part II: Biocompatibility

Hemolysis

None of the semifluorinated diethers induced a hemolysis either in the presence of serum or in the presence of isotonic saline solution.

Complement Activation (C3a)

Out of eight tested semifluorinated diethers, only two showed a mild [(CF₃CH₂O)₂(CH₂)₁₀] to moderate [(CF₃CH₂O)₂(CH₂)₇] activation of C3a complement (Tab. 2).

Table 2. Relative Complement Activation Induced by Semifluorinated Diethers

Test Compound	C3a Generation [% of Zymosan-Induced C3a]2^a
(CF3CH2O)2(CH2)3, n = 3	6.3	[0; 18.9]
(CF3CH2O)2(CH2)4, n = 3	4.7	[0; 14.1]
(CF3CH2O)2(CH2)5, n = 3	3.9	[0; 11.7]
(CF3CH2O)2(CH2)6, n = 3	5.1	[0; 13.8]
(CF3CH2O)2(CH2)7, n = 2	34.3	[33.9; 34.7]
(CF3CH2O)2(CH2)8, n = 2	0	[0; 0]
(CF3CH2O)2(CH2)9, n = 2	4.0	[0; 8.1]
(CF3CH2O)2(CH2)10, n = 2	19.6	[11.6; 27.1]

100% complement activation: C3a complement activation with zymosan; n: number of measurements.

^aConfidence interval.

Effects on Cytokine Production

Normally, mononuclear leukocytes are stimulated strongly by lipopolysaccharide and produce, among others, interleukin-1β. The semifluorinated diethers (CF₃CH₂O)₂(CH₂)_{3–5, 7, 8} caused almost a 100% suppression of this interleukin-1β production (Tab. 3). The other three diethers suppressed the interleukin-1β production to 50–60%. None of the semifluorinated diethers induced sizable inter- leukin-1β production by itself.

Table 3. Effects on Interleukin-1β after Incubation of Mononuclear Leukocytes (MNL) with Semifluorinated Diethers (Tc) Plus or Without Lipopolysaccharide (LPS)

Test Compound (tc)	IL1β Concentration [pg/mL] MNL + tc + LPS	Suppression of IL1β[%]4^b	Induction of IL1β [% of LPS Induced IL1β IL1β]
None, n = 5	2411 ± 855	—	—
(CF3CH2O)2(CH2)3, n = 2	Negative3^a	100 [100; 100]	none
(CF3CH2O)2(CH2)4, n = 2	262 ± 337	93 [86; 100]	none
(CF3CH2O)2(CH2)5, n = 3	Negative3^a	100 [100; 100]	none
(CF3CH2O)2(CH2)6, n = 3	1380 ± 1020	48 [8; 80]	1
(CF3CH2O)2(CH2)7, n = 3	79 ± 104	96 [90; 99]	none
(CF3CH2O)2(CH2)8, n = 2	27 ± 1	99 [98; 99]	none
(CF3CH2O)2(CH2)9, n = 2	868 ± 327	55 [27; 83]	1
(CF3CH2O)2(CH2)10, n = 2	1372 ± 1903	62 [26; 98]	none

n: number of measurements.

^aBeyond detection limit.

^bConfidence interval.

Cell Proliferation (³H-thymidine Incorporation)

The semifluorinated diethers (CF₃CH₂O)₂(CH₂)₃ and (CF₃CH₂O)₂(CH₂)₅ caused nearly a total inhibition of cell proliferation (Tab. 4). Results obtained with the Hela and Raji cell lines are comparable for three semifluorinated diethers. For the other four, the Hela cells seemed to be more sensitive to the toxic effects.

Table 4. Relative ³H-Thymidine Incorporation of Hela and Raji Cells After Incubation with Semifluorinated Diethers

Test Compound	Hela rel. ^3H-thymidine Incorporation [%]	Raji rel. ^3H-thymidine Incorporation [%]
(CF3CH2O)2(CH2)3, n = 1	1	1
(CF3CH2O)2(CH2)4, n = 1	22	62
(CF3CH2O)2(CH2)5, n = 1	2	3
(CF3CH2O)2(CH2)6, n = 1	4	96
(CF3CH2O)2(CH2)7, n = 1	n.d.	n.d.
(CF3CH2O)2(CH2)8, n = 1	58	100
(CF3CH2O)2(CH2)9, n = 1	41	45
(CF3CH2O)2(CH2)10, n = 1	34	100

100% ³H-thymidine incorporation: incubation of the cells with culture medium; n.d.: not determined; number of measurements: 1.

Cell Toxicity

All semifluorinated diethers had a marked toxic effect on the Hela and Raji cell lines (Tab. 5). Especially with (CF₃CH₂O)₂(CH₂)₃, only dead cells were found after a 24-h incubation. The results obtained for Hela and Raji cells are comparable except (CF₃CH₂O)₂(CH₂)₆.

Table 5. Cell Toxicity: Fraction of Dead Hela or Raji Cells After 24 h of Incubation

Test Compound	Hela Cells Fraction of Dead Cells After 24-h Incubation5^a	Raji Cells Fraction of Dead Cells After 24-h Incubation5^a
Medium, n = 4	8 [1; 12]	4 [0; 6]
Ringer's lactate, n = 4	10 [7; 15]	2 [0; 5]
(CF3CH2O)2(CH2)3, n = 2	100 [99; 100]	99 [99; 99]
(CF3CH2O)2(CH2)4, n = 2	73 [47; 98]	77 [56; 98]
(CF3CH2O)2(CH2)5, n = 1	86	100
(CF3CH2O)2(CH2)6, n = 2	73 [72; 74]	44 [20; 67]
(CF3CH2O)2(CH2)7, n = 2	88 [84; 91]	73 [65; 81]
(CF3CH2O)2(CH2)8, n = 3	74 [48; 93]	64 [52; 79]
(CF3CH2O)2(CH2)9, n = 4	53 [50; 59]	71 [67; 74]
(CF3CH2O)2(CH2)10, n = 4	54 [30; 82]	77 [66; 84]

n: number of independant measurements.

^aConfidence interval.

DISCUSSION

Our aim was the synthesis of substances having similar properties to perfluorocarbons such as chemical inertness and high oxygen solubility. In contrast, the substances should have a lower density. With their lower density it is possible that the damaging of the choroidal tissue observed with perfluorocarbons in the treatment of retinal detachment may be avoided. The symmetrical diethers we synthesized possess an oxygen solubility of 32–33 vol.-% O₂ [[11]] and reach therewith 72% of the oxygen solubility of perfluorodecalin (PFD), which is 45 vol.-% O₂ [[17]]. PFD is a typical perfluorocarbon that is widely used. The density of the symmetrical diethers is in the range of 1.1–1.2 g/cm³. The density is therefore near the density of blood and is much lower than the density of PFD or perfluorooctylbromide (PFOB), which is 1.9 g/cm³ [[2]]. In addition, the diethers were stable against nucleophils, strong bases, and strong oxidizers. The synthesized diethers have physical and chemical properties that make them attractive for application in the eye.

In addition to the physical and chemical tests, we conducted several in vitro biocompatibility tests. None of the substances caused hemolysis and only two substances showed mild activation of C3a complement. On the other hand, five of the eight diethers caused a nearly total suppression of interleukin-1β production, the other three to 50–60%. The cell proliferation tests showed, especially for the “short” semifluorinated diethers [(CF₃CH₂O)₂(CH₂)_3–6], a nearly total inhibition of cell proliferation. The results showed differences between the two cell lines, demonstrating that different cell lines may have different sensitivity toward toxic effects. Thus, it is important always to use more than one cell line to test biocompatibility. Finally, all diethers had a marked toxic effect on the Hela and Raji cell lines. Summing up, it may be said that all symmetrical diethers were positive in one or more tests and can be expected to be incompatible in vivo. “Short” semifluorinated diethers [(CF₃CH₂O)₂(CH₂)_3–6] seemed to be more toxic than those with more spacers. (CF₃CH₂O)₂(CH₂)₉ and (CF₃CH₂O)₂(CH₂)₁₀ seem to be the least toxic compounds. Further tests may be undertaken (e.g., with cells of the eye in vitro or even in animals), but we assume that further variation of the compounds will be necessary to generate better biocompatible derivatives.

After intensive search, we are not aware of any previous publication concerning the synthesis or the testing of symmetrical semifluorinated diethers. We are convinced that our system of establishing a biocompatibility profile will be useful in the future to check unknown compounds before they are used in animal experiments or even in humans.