Molecular Umbrella as a Nanocarrier for Antifungals

A molecular umbrella composed of two O-sulfated cholic acid residues was applied for the construction of conjugates with cispentacin, containing a “trimethyl lock” (TML) or o-dithiobenzylcarbamoyl moiety as a cleavable linker. Three out of five conjugates demonstrated antifungal in vitro activity against C. albicans and C. glabrata but not against C. krusei, with MIC90 values in the 0.22–0.99 mM range and were not hemolytic. Antifungal activity of the most active conjugate 24c, containing the TML–pimelate linker, was comparable to that of intact cispentacin. A structural analogue of 24c, containing the Nap-NH2 fluorescent probe, was accumulated in Candida cells, and TML-containing conjugates were cleaved in cell-free extract of C. albicans cells. These results suggest that a molecular umbrella can be successfully applied as a nanocarrier for the construction of cleavable antifungal conjugates.


Introduction
In the present SARS COVID-19 pandemic period, it is obvious that in the field of infectious diseases, special attention is focused on progress in the prevention of viral infections and antiviral chemotherapy. However, one cannot underestimate the fact that microbial resistance to antibiotics has emerged, and its spread worldwide has resulted in another significant threat to public health [1]. This challenge is also valid in the case of infections caused by human pathogenic fungi. Fungal micro-organisms from the Candida genus, especially Candida albicans and Candida glabrata but also an emerging pathogen Candida auris and filamentous fungi of Aspergillus spp., are etiological factors of many serious, often deadly infectious diseases, especially in immunocompromised patients [2]. C. albicans is considered the fourth most popular etiological agent of nosocomial infections worldwide [3]. There are several mechanisms of drug resistance of human pathogenic fungi, including those concerning alterations of drug transport across the microbial membranes, resulting from an impaired uptake of a drug molecule by membrane-located permeases or from its efflux, driven by multi-drug (MDR) energy-dependent drug transporters [4]. There is a generally accepted opinion on the urgent need for novel antifungal chemotherapeutics, especially these attacking new molecular targets. However, a number of drug candidates, including known inhibitors of intracellular enzymes identified as targets for antifungals, exhibit poor cellular penetration, owing to their hydrophilicity. One of the most promising strategies for overcoming this problem is the use of molecular carriers to ensure the delivery of enzymatic inhibitors to the intracellular target site [5], which is known as the "Trojan horse strategy". This approach is based on idea of conjugation of an active substance with a macromolecular or low molecular weight organic carrier easily penetrating the cell membrane [6]. After internalization, the conjugate remains intact or is cleaved, and the released active component can reach its intracellular target. Among the different organic nanocarriers proposed so far, an unique mechanism of internalization is characteristic for compounds known as "molecular umbrellas" [7]. They are "amphomorphic" compounds composed of two or more facial amphiphiles (mostly bile acids) that are connected to a central scaffold (spermine or spermidine). Molecular umbrellas were found capable of transporting certain hydrophilic molecules across liposomal and biological membranes [8] but have not been used so far as nanocarriers in antimicrobial conjugates. Interestingly, the conjugation of a molecular umbrella with known antifungal antibiotic Amphotericin B results in the "taming" of this drug, and as a consequence, improved selective toxicity [9,10].
In this work, rationally designed cleavable conjugates of the diwalled molecular umbrella with cispentacin, an inhibitor of Ile-tRNA Ile synthetase, have been tested as antifungal agents.

Rationale for Design of Conjugates
A basic rationale in this work was application of the molecular umbrella scaffold for the construction of conjugates with an inhibitor of intracellular fungal enzyme. The molecular umbrella was supposed to play the role of a nanocarrier responsible for the translocation of a conjugate through the fungal cytoplasmic membrane. Then, an enzymatic inhibitor should have been released from a conjugate intracellularly due to the presence of a cleavable linker, joining the nanocarrier and the cargo.
The simplest diwalled molecular umbrella was chosen as an optimal carrier, since it had been previously shown as the most effective umbrella component of conjugates with an antifungal antibiotic Amphotericin B, where its presence resulted in decreased mammalian toxicity [10]. We expected that the larger, tetra-or octawalled umbrellas may have problems with penetration through the fungal cell wall. Cispentacin, chosen as a cargo component, is an inhibitor of fungal Ile-tRNA Ile synthetase and as a consequence protein biosynthesis [11], which demonstrates antifungal activity [12]. This compound, for its polar character, does not penetrate biological membranes by simple diffusion but is taken up by the transmembrane transporter, proline permease [13]. For this reason, its activity is lower in L-proline containing complex media, where this amino acid competes with cispentacin for the permease and fungal resistance to this antifungal, and its analogue BAY 10-8888 is most often caused by decreased accumulation [14]. The need for a cleavable linker was obvious, since cispentacin interacts with its target exclusively as an intact molecule. In this work, two types of such linkers were employed. o-Dithiobenzylcarbamoyl moiety, previously successfully applied in the synthesis of a molecular umbrella-oligopeptide conjugate [8], contains a disulfide bond, which is potentially cleavable upon an intracellular thiol exchange reaction with the reduced glutathione. Nevertheless, this linker has not been employed so far for the construction of antimicrobial umbrella conjugates. The esterase-triggered TML ('trimethyl lock') [15], applied originally in antibacterial siderophore conjugates [16,17], has been for the first time used for the construction of potential antifungal conjugates.

Chemistry
Nine cleavable conjugates of a diwalled molecular umbrella with an antifungal agent, cispentacin, or with a fluorescent probe, Lys(Mca) (4-carboxymethyl-7-methoxycoumarin) or Nap-NH 2 (N-butyl-4-aminoethyl-1,8-naphtalimide), as cargo components were synthesized ( Figure 1). Seven out of nine conjugates contained the esterase-sensitive TML system, and the remaining two contained the o-dithiobenzylcarbamoyl linker. OSO   The synthesis of the esterase-labile TML linker started from pimelic 2a and suberic 2b acids, which were converted into corresponding cyclic anhydrides, using acetic anhydride. The resulting anhydrides 1b,c and commercially available succinic anhydride 1a were subsequently lysed with tert-butanol to dicarboxylic acid monoesters 3a-c in the manner described by Cisneros et al. [18].
The O-acylated TML systems 14a-c were prepared in a multistep synthesis presented in Scheme 1, being a modified version of the procedure described by Ji and Miller [16]. Condensation of the commercially available methyl 3-methylbut-2-enoate 4 with 3,5dimethylphenol 5 gave lactone 6, which after reduction with LiAlH4 resulted in the primary alcohol 7. Subsequently, the phenolic hydroxyl group of 7 was selectively protected with TBDMSCl (tert-butyldimethylchlorosilane). The protected alcohol 8 was acylated with appropriate carboxylic acid 3a-c by Steglich esterification. The primary hydroxyl groups of esters 9a-c were deprotected under mildly acidic conditions, resulting in alcohols 10a-c, which underwent subsequent oxidation to corresponding aldehydes 11a-c and then to carboxylic acids 12a-c. The formed carboxylic group was protected with benzyl bromide, thus forming triesters 13a-c. The subsequent deprotection of tert-butyl esters with trifluoroacetic acid gave carboxylic acids 14a-c (Scheme 1). The synthesis of the esterase-labile TML linker started from pimelic 2a and suberic 2b acids, which were converted into corresponding cyclic anhydrides, using acetic anhydride. The resulting anhydrides 1b,c and commercially available succinic anhydride 1a were subsequently lysed with tert-butanol to dicarboxylic acid monoesters 3a-c in the manner described by Cisneros et al. [18].
The O-acylated TML systems 14a-c were prepared in a multistep synthesis presented in Scheme 1, being a modified version of the procedure described by Ji and Miller [16]. Condensation of the commercially available methyl 3-methylbut-2-enoate 4 with 3,5dimethylphenol 5 gave lactone 6, which after reduction with LiAlH 4 resulted in the primary alcohol 7. Subsequently, the phenolic hydroxyl group of 7 was selectively protected with TBDMSCl (tert-butyldimethylchlorosilane). The protected alcohol 8 was acylated with appropriate carboxylic acid 3a-c by Steglich esterification. The primary hydroxyl groups of esters 9a-c were deprotected under mildly acidic conditions, resulting in alcohols 10a-c, which underwent subsequent oxidation to corresponding aldehydes 11a-c and then to carboxylic acids 12a-c. The formed carboxylic group was protected with benzyl bromide, thus forming triesters 13a-c. The subsequent deprotection of tert-butyl esters with trifluoroacetic acid gave carboxylic acids 14a-c (Scheme 1). Formation of the molecular umbrella structure on the prepared TML linker started with the conjugation of carboxylic acids 14a-c with Boc-protected spermidine 16, as presented in Scheme 2. The synthesis of protected spermidine was accomplished in a one-step reaction of spermidine 15 with 2 equivalents of protecting agent, Boc-ON. The resulting amine 16 was subsequently acylated with NHS (N-hydroxysuccinimide)-activated TML linkers 17a-c (NHS/DCC method), and then, the benzyl protecting group was removed by hydrogenolysis on Pd/C catalyst. The resulting carboxylic acids 19a-c were universal building blocks for further conjugation with cargo molecules.
Formation of the molecular umbrella structure on the prepared TML linker started with the conjugation of carboxylic acids 14a-c with Boc-protected spermidine 16, as presented in Scheme 2. The synthesis of protected spermidine was accomplished in a one-step reaction of spermidine 15 with 2 equivalents of protecting agent, Boc-ON. The resulting amine 16 was subsequently acylated with NHS (N-hydroxysuccinimide)-activated TML linkers 17a-c (NHS/DCC method), and then, the benzyl protecting group was removed by hydrogenolysis on Pd/C catalyst. The resulting carboxylic acids 19a-c were universal building blocks for further conjugation with cargo molecules. Formation of the molecular umbrella structure on the prepared TML linker started with the conjugation of carboxylic acids 14a-c with Boc-protected spermidine 16, as presented in Scheme 2. The synthesis of protected spermidine was accomplished in a one-step reaction of spermidine 15 with 2 equivalents of protecting agent, Boc-ON. The resulting amine 16 was subsequently acylated with NHS (N-hydroxysuccinimide)-activated TML linkers 17a-c (NHS/DCC method), and then, the benzyl protecting group was removed by hydrogenolysis on Pd/C catalyst. The resulting carboxylic acids 19a-c were universal building blocks for further conjugation with cargo molecules.  The antifungal agent, cispentacin, was connected to the 'trimethyl lock' linker by amide bond formation, which was accomplished by the activation of carboxylic acids 19a-c with TDBTU (O-(3,4-dihydro-4-oxo-1,2,3-benzotriazin-3-yl)-N,N,N ,N -tetramethyluronium tetrafluoroborate) followed by aminolysis with cispentacin (Scheme 3). The resulting amides 23a-c were deprotected under acidic conditions of trifluoroacetic acid and used for the ultimate formation of the molecular umbrella. For that purpose, deoxycholic acid 20a and cholic acid 22b were converted into appropriate active esters 22a-c. The formation of ester 22c was straightforward and was accomplished using the NHS/DCC technique. For the preparation of active esters 22a,b, cholic and deoxycholic acids were previously converted into sulfate derivatives 21a,b by esterification with a sulfur trioxide/pyridine complex. Then, salts 21a,b were converted to active esters 22a,b with TDBTU (Scheme 3). Active esters 22a,b were used for the acylation of spermidine terminal amino groups, which resulted in final molecular umbrella-cispentacin conjugates 24a-d, differing by the length of the "trimethyl lock" containing linkers and the presence or absence of sulfate groups in the cholic acid "walls".
for the ultimate formation of the molecular umbrella. For that purpose, deoxycholic acid 20a and cholic acid 22b were converted into appropriate active esters 22a-c. The formation of ester 22c was straightforward and was accomplished using the NHS/DCC technique. For the preparation of active esters 22a,b, cholic and deoxycholic acids were previously converted into sulfate derivatives 21a,b by esterification with a sulfur trioxide/pyridine complex. Then, salts 21a,b were converted to active esters 22a,b with TDBTU (Scheme 3). Active esters 22a,b were used for the acylation of spermidine terminal amino groups, which resulted in final molecular umbrella-cispentacin conjugates 24a-d, differing by the length of the "trimethyl lock" containing linkers and the presence or absence of sulfate groups in the cholic acid "walls".
for the ultimate formation of the molecular umbrella. For that purpose, deoxycholic acid 20a and cholic acid 22b were converted into appropriate active esters 22a-c. The formation of ester 22c was straightforward and was accomplished using the NHS/DCC technique. For the preparation of active esters 22a,b, cholic and deoxycholic acids were previously converted into sulfate derivatives 21a,b by esterification with a sulfur trioxide/pyridine complex. Then, salts 21a,b were converted to active esters 22a,b with TDBTU (Scheme 3). Active esters 22a,b were used for the acylation of spermidine terminal amino groups, which resulted in final molecular umbrella-cispentacin conjugates 24a-d, differing by the length of the "trimethyl lock" containing linkers and the presence or absence of sulfate groups in the cholic acid "walls".
The synthesis of molecular umbrella conjugates with a o-dithiobenzoylcarbamoyl linker was inspired by the research work of Jing et al. [19]. As shown in Scheme 5, the synthesis began with commercially available active diester 28, which was converted to a dimeric structure 29 by amide formation with 2 equivalents of Boc 2 -protected spermidine 16.
Intermediate 29 was subsequently cleaved to corresponding thiol 30, using TCEP (tris(2carboxyethyl)phosphine) as a reducing agent. Then, a thiol group was activated with asymmetric disulfide 31, affording a mixed disulfide 32, which underwent an activation reaction with DSC (N,N -disuccinimidyl carbonate). Active ester 33 reacted with cispentacin, which resulted in urethane bond formation in derivative 34. As in the case of TML-containing conjugates, the Boc-protecting groups were removed with TFA, and the resulting primary amino groups were acylated with cholic acid active ester 22b, leading to the final conjugate 35. The synthesis of molecular umbrella conjugates with a o-dithiobenzoylcarbamoyl linker was inspired by the research work of Jing et al. [19]. As shown in Scheme 5, the synthesis began with commercially available active diester 28, which was converted to a dimeric structure 29 by amide formation with 2 equivalents of Boc2-protected spermidine 16. Intermediate 29 was subsequently cleaved to corresponding thiol 30, using TCEP (tris(2-carboxyethyl)phosphine) as a reducing agent. Then, a thiol group was activated with asymmetric disulfide 31, affording a mixed disulfide 32, which underwent an activation reaction with DSC (N,N′-disuccinimidyl carbonate). Active ester 33 reacted with cispentacin, which resulted in urethane bond formation in derivative 34. As in the case of TML-containing conjugates, the Boc-protecting groups were removed with TFA, and the resulting primary amino groups were acylated with cholic acid active ester 22b, leading to the final conjugate 35.

Three Out of Five Umbrella: Cispentacin Conjugates Demonstrate Antifungal In Vitro Activity
The antifungal in vitro activity of conjugates 24a-d and 35 was determined by the serial two-fold dilution method, using the 96-well microtiter plates, in two growth media, RPMI-1640 and YNB-AS, against seven yeast species from the Candida genus. The MIC50 The synthesis of molecular umbrella conjugates with a o-dithiobenzoylcarbamoyl linker was inspired by the research work of Jing et al. [19]. As shown in Scheme 5, the synthesis began with commercially available active diester 28, which was converted to a dimeric structure 29 by amide formation with 2 equivalents of Boc2-protected spermidine 16. Intermediate 29 was subsequently cleaved to corresponding thiol 30, using TCEP (tris(2-carboxyethyl)phosphine) as a reducing agent. Then, a thiol group was activated with asymmetric disulfide 31, affording a mixed disulfide 32, which underwent an activation reaction with DSC (N,N′-disuccinimidyl carbonate). Active ester 33 reacted with cispentacin, which resulted in urethane bond formation in derivative 34. As in the case of TML-containing conjugates, the Boc-protecting groups were removed with TFA, and the resulting primary amino groups were acylated with cholic acid active ester 22b, leading to the final conjugate 35.

Three Out of Five Umbrella: Cispentacin Conjugates Demonstrate Antifungal In Vitro Activity
The antifungal in vitro activity of conjugates 24a-d and 35 was determined by the serial two-fold dilution method, using the 96-well microtiter plates, in two growth media, RPMI-1640 and YNB-AS, against seven yeast species from the Candida genus. The MIC50

Three Out of Five Umbrella: Cispentacin Conjugates Demonstrate Antifungal In Vitro Activity
The antifungal in vitro activity of conjugates 24a-d and 35 was determined by the serial two-fold dilution method, using the 96-well microtiter plates, in two growth media, RPMI-1640 and YNB-AS, against seven yeast species from the Candida genus. The MIC 50 and MIC 90 values, i.e., concentrations at which cell growth was inhibited in 50% and 90%, respectively, were measured and compared to those of the intact cispentacin. The concentration values presented in Tables 1 and 2 are expressed in mM units to allow the comparison of the compounds' effectiveness at the molecular level. For data expressed in µg mL −1 units, such comparison is not possible, since the molecular masses of conjugates (≈1755-2114 D range) are 14-16-fold higher than that of cispentacin (128.3 D). Table 1. Antifungal in vitro activity of cispentacin and its conjugates 24a-d and 35. MIC 50 and MIC 90 values (mM) were determined by the serial two-fold dilution method, using the 96-well microtiter plates, in RPMI-1640 medium. CisP-cispentacin. Data >> 1 mM means than no growth inhibition was observed at 1 mM.  Data presented in Table 1 indicate that conjugate 24c demonstrated a growth inhibitory effect in RPMI-1640 medium against all Candida spp. Tested, except for C. krusei. The MIC 50 and MIC 90 values of this compound were comparable to those of cispentacin, although C. glabrata was slightly more susceptible to 24c than to cispentacin, and the same was found for some C. albicans clinical isolates. Compounds 24a, 24b, 24d, and 35 were devoid of antifungal activity in this medium. Table 2. Antifungal in vitro activity of cispentacin and its conjugates 24a-d and 35. MIC 50 and MIC 90 values (mM) were determined by the serial two-fold dilution method, using the 96-well microtiter plates, in YNB-AS medium. CisPcispentacin. Data >> 1 mM means than no growth inhibition was observed at 1 mM, while >>1 mM indicates that at 1 mM, some growth inhibition (10-15%) was observed.  As shown in Table 2, in minimal YNB-AS medium, a growth inhibitory effect was found for 24c against C. albicans and C. glabrata, but some activity was also observed for 24a and 24b against C. albicans.
The MIC 50 and MIC 90 values of cispentacin in minimal YNB-AS medium were two to three times lower than those in RPMI-1640. The latter medium contains a mixture of proteinogenic amino acids, including L-proline. This amino acid competes with cispentacin for proline permease, thus lowering the growth inhibitory activity of this antifungal. In the case of 24c conjugate, the difference between its MIC 50 and MIC 90 values measured in YNB-AS and in RPMI-1640 was much smaller, if any. On the other hand, the 24a and 24b conjugates were inactive in RPMI-1640 but demonstrated some activity in YNB-AS.
It is not clear which components of the RPMI-1640, not present in YNB-AS, affected the antifungal activity of these two conjugates.
A comparison of the growth inhibitory activities of cispentacin and its conjugates with the diwalled molecular umbrella against clinical FLC (fluconazole)-resistant and FLC-susceptible C. albicans strains revealed no substantial difference for 24c and slightly lower activity of cispentacin against FLC-resistant B4 and Gu5 than against their respective FLC-susceptible counterparts B3 and Gu4. This result suggests than cispentacin could be a substrate for drug efflux pumps, Cdr1p/Cdr2p overexpressed in Gu5 and for Mdr1p present in B4, whereas the conjugates, especially 24c, are not.
No antifungal activity in both growth media was found for conjugates of molecular umbrella with fluorescent probes, 26a,b, 27, and 37.

Molecular Umbrella: Cispentacin Conjugates Are Not Hemolytic
Cispentacin, all five molecular umbrella/cispentacin conjugates 24a-d and 35, as well as four conjugates with fluorescent probes 26a,b, 27, and 37 were tested for hemolytic activity against human erythrocytes. Conjugates 24a-d, 26a,b, 27, 35, and 37 were tested in the 2000-16 µg mL −1 range (2000 µg mL −1 corresponds to approximately 1 mM concentration) and cispentacin in the 200-1.6 µg mL −1 range. Not more than 7.5% hemolysis at the higher concentration of each compound was found, which means that all conjugates tested are not hemolytic.
2.5. Molecular Umbrella: TML:Nap-NH 2 Conjugate 27 Is Accumulated in C. albicans and C. glabrata but Not in C. krusei Cells Four conjugates of a molecular umbrella with fluorescent probes, structural analogs of compounds 24a-c and 35, were constructed in order to use them for studies on the internalization of molecular umbrella conjugates in Candida cells. Three of them, namely 26a, 26b, and 37, contained Mca linked through L-lysine or 2,4-diaminobutanoic acid to the TML system and in 27, the fluorescent probe was Nap-NH 2 , which was connected directly to TML.
For uptake studies, Candida cells were treated with a molecular umbrella/fluorescent probe conjugate, and intracellular probe accumulation was monitored by fluorescence microscopy, with λ exc = 438 nm for Nap-NH 2 and λ exc = 350 nm for Lys(Mca).
The results of microscopic examination of 27 uptake by Candida cells are shown in Figure 2. This conjugate was effectively accumulated in a time-dependent fashion in C. albicans and C. glabrata cells but not in C. krusei. No fluorescent probe accumulation was observed with conjugates 26a, 26b, and 37 (images not shown).

Conjugates Containing TML Are Cleaved in the Model System and in Cell-Free Extract
Conjugates 24a-d, containing the TML system, were tested for susceptibility to enzymatic cleavage in the model system with pig liver esterase and in cell-free extract prepared from C. albicans cells. Conjugate 35 containing the o-dithiobenzylcarbamoyl linker was tested in the model system with glutathione and in C. albicans cell-free extract.
Samples collected from incubation mixtures at time intervals were subjected to HPLC-MS analysis. Results of these analyses of mixtures containing products of digestion of conjugates 24c and 24d with pig esterase are shown in Figure 3A,B. In both cases, a peak with time-dependent increasing intensity appeared at a location corresponding to that of the intact cispentacin. Presence of the released cispentacin was confirmed by MS spectrum, where a signal at m/z = 128.3 is clearly shown. The respective peak was not present in chromatograms of mixtures resulting from pig esterase treatment of 26a and 26b but was found in the chromatogram of post-reaction mixture resulting from glutathione action on 35 (not shown).

Conjugates Containing TML Are Cleaved in the Model System and in Cell-Free Extract
Conjugates 24a-d, containing the TML system, were tested for susceptibility to enzymatic cleavage in the model system with pig liver esterase and in cell-free extract prepared from C. albicans cells. Conjugate 35 containing the o-dithiobenzylcarbamoyl linker was tested in the model system with glutathione and in C. albicans cell-free extract.
Samples collected from incubation mixtures at time intervals were subjected to HPLC-MS analysis. Results of these analyses of mixtures containing products of digestion of conjugates 24c and 24d with pig esterase are shown in Figure 3A,B. In both cases, a peak with time-dependent increasing intensity appeared at a location corresponding to that of the intact cispentacin. Presence of the released cispentacin was confirmed by MS spectrum, where a signal at m/z = 128.3 is clearly shown. The respective peak was not present in chromatograms of mixtures resulting from pig esterase treatment of 26a and 26b but was found in the chromatogram of post-reaction mixture resulting from glutathione action on 35 (not shown).  Figure 4A,B.
Peaks with time-dependent increasing intensity, at locations roughly corresponding to those of the intact cispentacin, are present in both chromatograms. Surprisingly, in the The analogous HPLC-MS analyses were performed for samples collected at time intervals from mixtures consisting of conjugates 24a-d or 35 dissolved in the cell-free extract of C. albicans cells. Results of analyses of mixtures containing conjugates 24c and 24d are shown in Figure 4A,B.
Peaks with time-dependent increasing intensity, at locations roughly corresponding to those of the intact cispentacin, are present in both chromatograms. Surprisingly, in the MS spectra, there are not signals with m/z = 128.3. Instead, the strong signals at m/z = 145.1 or 145.2 are present. Most likely, these signals may be attributed to any cispentacin derivative formed in the cell-free extract. In the control experiment, cispentacin was incubated in the C. albicans cell-free extract, but the MS analysis did not reveal the formation of any compound with m/z = 145.1 or 145.2.
Neither cispentacin nor the m/z 145.1/145.2 compound was found in the reaction mixtures containing conjugates 24a, 24b, or 35. It may suggest that these conjugates are not cleaved in C. albicans cell-free extract.

Discussion
Among the five rationally designed conjugates of a diwalled molecular umbrella with cispentacin, joined through the cleavable linker, only one, namely the TML-pimelate system containing 24c, exhibited antifungal in vitro activity in RPMI-1640 medium under CLSI-recommended conditions. Two analogs of this conjugate, containing the much shorter TML-succinate linker, 24a and 24b, appeared active only in minimal YNB-AS medium, while conjugates 24d and 35 (containing the o-dithiobenzylcarbamoyl linker) were inactive in both media. Lack of the antifungal activity of 35 in RPMI-1640 medium could be at least in part explained by the presence of reduced glutathione at 1 mg L −1 in this medium; however, YNB-AS does not contain any thiol compounds. Rather unexpectedly, conjugate 35 was not cleaved in the cytoplasmic extract of C. albicans, although cleavage was observed in the model system with 5 mM glutathione. No cleavage in cell-free extract was also found for TML-containing conjugates 24a and 24b. However,

Discussion
Among the five rationally designed conjugates of a diwalled molecular umbrella with cispentacin, joined through the cleavable linker, only one, namely the TML-pimelate system containing 24c, exhibited antifungal in vitro activity in RPMI-1640 medium under CLSI-recommended conditions. Two analogs of this conjugate, containing the much shorter TML-succinate linker, 24a and 24b, appeared active only in minimal YNB-AS medium, while conjugates 24d and 35 (containing the o-dithiobenzylcarbamoyl linker) were inactive in both media. Lack of the antifungal activity of 35 in RPMI-1640 medium could be at least in part explained by the presence of reduced glutathione at 1 mg L −1 in this medium; however, YNB-AS does not contain any thiol compounds. Rather unexpectedly, conjugate 35 was not cleaved in the cytoplasmic extract of C. albicans, although cleavage was observed in the model system with 5 mM glutathione. No cleavage in cell-free extract was also found for TML-containing conjugates 24a and 24b. However, these conjugates were also found resistant to enzymatic cleavage by pig esterase in the model system. Most probably, the succinic acid link between spermidine scaffold and TML appeared to be short, and due to the close vicinity of the molecular umbrella-spermidine scaffold and TML, enzyme access to the ester bond in TML system is hindered. It seems that pimelic acid could be the optimal dicarboxylic linker joining the TML system with the spermidine scaffold. Conjugate 26d, containing suberic acid, which is only one methylene unit longer than pimelic acid, was not active as an antifungal, which was apparently due to the problems with internalization, since it was found cleavable in the cell-free extract. Notably, pimelate was previously found as the optimal linker in non-cleavable molecular umbrella-Amphotericin B conjugates [10].
Among the three Candida species tested, C. krusei appeared resistant to all conjugates, including 24c. Since conjugate 27, the fluorescent analogue of 24c, was not accumulated by C. krusei cells, one may suspect that 24c is also not taken up by these cells. We have no explanation for this phenomenon; nevertheless, it shows that the molecular umbrella may not by an universal molecular nanocarrier.
From the results of HPLC-MS analysis of metabolism of conjugates 24c and 24d in C. albicans cell-free extract, it is clear that the product of a possible cleavage, with RT similar to that of cispentacin but with 17 D higher MW, was formed, while such a product was not observed when intact cispentacin was incubated in the C. albicans cell-free extract. In this context, it is worth mentioning that there have not been any previous reports on the possible modification of cispentacin in the cytosol of fungal cells. On the other hand, it was found that the close structural analogue of cispentacin, BAY 10-8888, known also as Icofungipen or PLD-118, was not metabolized by liver microsomes under in vitro conditions [20]. Undoubtedly, further studies are necessary to allow a correct interpretation of this finding, and any speculation on the identity of a possible cispentacin metabolite makes no sense.
So far, molecular umbrellas have been tested as possible molecular nanocarriers exclusively in liposomal or mammalian cell systems [8,21] with no attempts concerning microbial, cell wall-containing cells. The presence of the cell wall surrounding the cytoplasmic membrane might theoretically constitute a physical barrier for molecules as large as molecular umbrellas and their conjugates. In this respect, it is worth mentioning that small but not large dendrimers were used as nanocarriers for some antimicrobials [22] and, on the other hand, tetra-and octawalled molecular umbrellas were not effective as components of conjugates with Amphotericin B [23]. Now, we have been able to demonstrate that at least conjugates incorporating a diwalled molecular umbrella are able to transgress this obstacle in Candida cells. This finding opens the possibility of further attempts aimed at the construction of other conjugates more active than those described in this work, possibly with cargo molecules other than cispentacin.

General
All solvents and reagents were used as obtained from commercial sources. 1 H NMR and 13 C NMR spectra were obtained at 500 MHz Varian Unity Plus spectrometer, Varian Medical Systems, Darmstadt, Germany. and the deuterated solvents were used as internal locks. FTIR spectra were obtained with a Thermo Electron Nicolet 8700 spectrometer, Thermo Fisher Scientific, Waltham, MA, USA. High-resolution mass spectra were obtained using the Aqilent Technologies 6540 UHD Accurate-Mass Q-TOF LC/MS mass spectrometer, Waldbronn, Germany. Melting points were determined on a melting point apparatus equipped with a thermometer and were uncorrected. Column chromatography was performed with silica gel (0.040-0.063 mm) by using the indicated solvent systems. The following abbreviations are used in reporting NMR data: s-singlet, brs-broad singlet, d-doublet, dd-doublet of doublets, t-triplet, q-quartet, qt-quintet.

Syntheses
Dicarboxylic Acid Anhydrides-General Procedure (1b,c), First, 19 mmol of a dicarboxylic acid was dissolved in 50 mL of acetic anhydride and refluxed for 4 h. The solvents were removed under reduced pressure, obtaining 18 mmol of a cyclic anhydride.
Pimelic Anhydride 1b, Starting from 3 g (19 mmol) of pimelic acid, 2.64 g (18 mmol, 95%) of pimelic anhydride 1b was obtained as a light-beige, low-melting solid. 1  Mono-tert-butyl esters (3a-c)-General Procedure, First, 20 mmol of carboxylic acid anhydride, 2 mmol of DMAP, and 5.88 mmol of NHS were suspended in 75 mL of toluene. Subsequently, 40 mmol of tert-butanol and 6 mmol of TEA were added, and the resulting mixture was refluxed for 24 h in an oil bath. The mixture was cooled to room temperature; then, 75 mL of AcOEt was added. The resulting solution was washed with 1M HCl (aq) (3 × 70 mL) and with brine (3 × 70 mL), respectively. The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure. The residue was purified by liquid column chromatography or crystallization.

2-(3-hydroxy-1,1-dimethylpropyl)-3,5-dimethylphenol 7,
To a stirred suspension of 4 g (0.104 mol) of LiAlH 4 in 100 mL of dry THF, 5.3 g (26 mmol) of lactone 6 was added in small portions. Once the addition was completed, the reaction mixture was stirred at room temperature for 3 h. The excess of unreacted LiAlH 4 was decomposed by adding AcOEt, MeOH, and water, respectively. The slurry was filtered off under reduced pressure, and the filtrate was concentrated under reduced pressure. The residue was dissolved in 100 mL of dichloromethane and dried over anhydrous MgSO 4 . After the removal of desiccant and concentration of filtrate under reduced pressure, the residue was recrystallized from dichloromethane. .05 mmol of DMAP in 100 mL of dry dichloromethane was cooled in an ice bath to 0 • C, and then, 30.5 mmol of DCC dissolved in 20 mL of dry dichloromethane was added dropwise. The mixture was stirred at room temperature for 48 h. The precipitate of DCU was filtered off, and the filtrate was concentrated under reduced pressure. The residue was dissolved in 100 mL of chloroform and washed with saturated NaHCO 3(aq) (2 × 50 mL), 5% NaHSO 4(aq) (2 × 50 mL), and water (2 × 50 mL), respectively. The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure. The residue was purified by liquid column chromatography, using a mixture of solvents hexanes/AcOEt, 9/1, v/v as a mobile phase.  13 13 13

3-[2-(O-acyloyl)hydroxyl-4,6-dimethyl]phenyl-3-methylbutanoic acid (12a-c)-General
Procedure, To a stirred, ice-cold mixture of 2.48 mmol of aldehyde, 100 µL of 50% H 2 O 2 and 80 mg of NaH 2 PO 4 in 3 mL of MeCN/H 2 O, 3/1, v/v, 450 mg of NaClO 2 in 2 mL of water was added dropwise. The reaction mixture was stirred at 0 • C for 1 h and then warmed to room temperature. The excess of oxidizing reagents was decomposed by a dropwise addition of saturated aqueous solution Na 2 S 2 O 3 . Subsequently, the mixture was acidified with 3M HCl to pH 2 and extracted with AcOEt (3 × 30 mL). The organic layer was washed with water (30 mL), brine (2 × 30 mL), and water (30 mL), respectively. The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure. The product was purified by liquid column chromatography using a mixture of solvents hexanes/AcOEt/AcOH.  30 (m, 11H). 13  Benzyl 3-{2-[O-(4-tertbutoxyacyloyl)]hydroxyl-4,6-dimethyl}phenyl-3-methylbutanoate (13a-c)-General Procedure, To a mixture of 2.64 mmol of carboxylic acid dissolved in 20 mL of dry DMF, 5.28 mmol of potassium bicarbonate was added, and the resulting suspension was allowed to stir at room temperature for 15 min. Subsequently, 4 mmol of benzyl bromide was added in one portion, and then, the reaction mixture was allowed to stir at 40 • C for 3 h. The mixture was cooled to room temperature, and 30 mL of 5% solution of NaHCO 3(aq) was added. The resulting mixture was extracted with ethyl acetate (3 × 50 mL), and then, the organic layer was washed with 5% solution of NaHCO 3(aq) (2 × 50 mL) and brine (2 × 50 mL), respectively. The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure. The residue was purified by liquid column chromatography using a mixture of solvents hexanes/AcOEt, 8/2, v/v as a mobile phase.  13 13 13  -dimethyl]phenyl-3-methylbutanoate (14a-c)-General Procedure, First, 2.13 mmol of tert-butyl ester was dissolved in 20 mL of a mixture of DCM/TFA, 3/1, v/v and allowed to stir at room temperature for 1 h. The solvents were removed under reduced pressure, and the residue was purified by liquid column chromatography, using a mixture of solvents hexanes/AcOEt/AcOH, 70/30/1, v/v/v as a mobile phase.  13 13  Boc 2 -spermidine-'trimethyl lock' benzyl ester building block (18a-c)-General Procedure, To the solution of 2.42 mmol of carboxylic acid 14a-c and 3.06 mmol of NHS in 20 mL of dry DCM, 4.34 mmol of DCC dissolved in 5 mL of dry DCM was added dropwise, and then, the mixture was allowed to stir at room temperature for 24 h. The precipitate of DCU was filtered off under reduced pressure, and the filtrate was diluted with 50 mL of DCM. Subsequently, the mixture was washed with water (2 × 30 mL), saturated solution of NaHCO 3(aq) (2 × 30 mL), and water (2 × 30 mL), respectively. The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure. The residue was dissolved in 50 mL of dry DCM, and 6.72 mmol of DIPEA and 3.2 mmol of Boc 2 -spermidine 16 were added. The mixture was allowed to stir at room temperature for 5 h; then, 20 mL of DCM was added, and the resulting solution was washed with brine (2 × 30 mL), 5% solution of NaHSO 4(aq) (2 × 30 mL), and brine (2 × 30 mL), respectively. The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure. The residue was purified by liquid column chromatography, using a mixture of solvents hexanes/AcOEt, 4/6, v/v as a mobile phase.   (3.03 mmol) of DEPBT were added, respectively. The mixture was stirred at room temperature for 2 h and then concentrated under reduced pressure. The oily residue was treated with MeCN and the formed precipitate was filtered off, obtaining 1.35 g (1.57 mmol, 65%) of crude active ester 22a as a light-yellow solid, which was used in the next step without further purification.
Tetrasodium O,O ,O -tris(sulfate)cholate active ester 22b, Active ester 22b was prepared in a similar manner as 22a. Starting from 1.50 g (2.04 mmol) of cholic acid derivative 21b, 1.16 g (1.35 mmol, 66%) of active ester 22b was obtained as a light-yellow solid, which was used in the next step without further purification.
Cholic acid active ester 22c, First, 15 g (37 mmol) of cholic acid was dissolved in 250 mL of dry THF, which was followed by the addition of 4.26 g (37 mmol) of NHS. Subsequently, 9.16 g (44 mmol) of DCC dissolved in 30 mL of dry THF was added dropwise. The mixture was stirred at room temperature for 24 h; then, the precipitated DCU was filtered off under reduced pressure, and the filtrate was concentrated under reduced pressure. The residue was dissolved in 300 mL of CHCl 3 and washed with a saturated solution of NaHCO 3(aq) (2 × 200 mL) and water (2 × 200 mL). The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure, obtaining 10.61 g (21 mmol, 57%) of crude active ester 22c, which was used in the next step without further purification.
Molecular umbrella-'trimethyl lock'-cargo conjugates (24a-d)-General Procedure, To a solution of 2.17 mmol of carboxylic acid 19a-c in 20 mL of dry DMF, 568 µL of DIPEA and 3.26 mmol of DEPBT were added, respectively. The mixture was stirred at room temperature for 2 h. Then, 100 mL of CHCl 3 was added, and the resulting mixture was washed with 1 M solution of HCl (aq) (2 × 75 mL), brine (2 × 75 mL), saturated solution of NaHCO 3(aq) (2 × 75 mL), and brine (2 × 75 mL). The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure. The crude active ester was roughly purified by liquid column chromatography, using a mixture of solvents hexanes/AcOEt, 4/6, v/v as a mobile phase. Then, 0.70 mmol of active ester was dissolved in 20 mL of dry DMF, and then 1.1 mL of DIPEA and 1.05 mmol of cispentacin were added, respectively. The mixture was stirred at room temperature for 2 h. Subsequently, 100 mL of CHCl 3 was added, and the resulting solution was washed with 1 M solution of HCl (aq) (3 × 50 mL) and brine (3 × 50 mL). The organic layer was dried over anhydrous MgSO 4 , the desiccant was filtered off, and the filtrate was concentrated under reduced pressure, obtaining crude product 23a-c. Product 23a-c was dissolved in a mixture of DCM/TFA, 3/1, v/v and allowed to stir at room temperature for 1 h. Then, the solvents were evaporated under reduced pressure, obtaining the crude deprotection product which was used in the next step without further purification. To the solution of 0.30 mmol of deprotected spermidine derivative in dry DMF, 627 µL of DIPEA and 0.60 mmol of active ester 22a or 22b were added, respectively. The mixture was stirred at room temperature for 24 h; then, the solvents were evaporated under reduced pressure, and the residue was treated with MeCN. The precipitate was collected by filtration under reduced pressure and purified by liquid column chromatography.
Molecular umbrella-cispentacin conjugate 24a, Starting from 700 mg (1. Values of MIC 50 and MIC 90 were read from the graphs of A 660 vs. drug concentration and corresponded to the interpolated values of drug concentration at which A 660 was 50% or 10% of that measured for the drug-free control.

Hemolytic Activity Determination
Red blood cell concentrates were kindly provided by the Regional Center for Blood Donation and Blood Treatment in Gdańsk. The hemolytic activity determination was carried out by the serial dilution method, according to the procedure described earlier [26]. Briefly, human erythrocytes were suspended in saline to give a suspension of 2 × 10 7 cells mL −1 (hemocytometer count). The stock 1 mg mL −1 solutions of conjugates were prepared in DMSO, and 50 µL aliquots of serial two-fold dilutions were placed in Eppendorf tubes. Tubes containing 50 µL of DMSO and 50 µL of 2% aqueous Triton X-100 solution were included as a negative and positive control, respectively. To each tube, 950 µL of the erythrocyte suspension was added and mixed by inversion to give the final concentrations of compounds tested in the 200-0.4 µg mL −1 range. The samples were incubated at 37 • C for 30 min; then, they were mixed by inversion and centrifuged (1700× g, 5 min, 4 • C). The concentration of hemoglobin in supernatants obtained after the centrifugation of erythrocytes suspension was determined by measuring the absorbance at wavelength λ = 540 nm (A 540 sample ). Absorbance of the negative (A 540 DMSO ) and the positive (A 540 0.1%Triton X−100 ) controls was also measured. The percent of hemolysis at a given compound concentration/EH (%)/was calculated as follows: EH (%) = ((A 540 sample − A 540 DMSO )/(A 540 0.1%Triton X−100 − A 540 DMSO )) × 100.
The EH 50 values for each compound were calculated with the GraphPad Prism software as an interpolated concentration of compound, for which the A 540 value is exactly 50% of the A 540 value measured for the positive control sample.

Preparation of C. albicans Cell-Free Extracts
C. albicans cells were harvested from the culture in YPD medium in the logarithmic phase of growth and washed with PBS, pH 7.4. Then, cells were suspended in a minimal amount of PBS to an optical density OD 660 ≈ 1.5. A sample of 1.5 mL of the cell suspension was put into the Lysing Matrix D, 2 mL tubes containing 1.4 mm ceramic spheres (MP Biomedicals). The tubes were vortexed 5 times for 2 min intervals, with 2 min breaks for cooling in the ice/water bath. Finally, the mixtures were centrifuged (10,000× g, 4 • C, 20 min), and the supernatant was pushed through the syringe filter, 0.22 µm pore size, to remove unbroken cells. The thus-obtained cell-free extract was kept at 4 • C and used no later than 5 h after preparation.

HPLC-MS Analysis of Conjugate Cleavage
Analysis of cleavage of the conjugates was performed in two experimental systems: (A) model system, in the presence of pig liver esterase or glutathione, reduced; (B) in C. albicans cell-free extract.
System A Mixtures containing a given conjugate at the initial concentration of 150 µM and pig liver esterase, 5 mg mL −1 or 10 mM glutathione, reduced, in a total volume of 200 µL were incubated at 30 • C. Samples of 50 µL were collected at zero time, after 30 min, and after 120 min and subjected to HPLC-MS analysis.
System B A given conjugate was dissolved in 4 mL of C. albicans cell-free extract to the initial concentration of 150 µM. Mixtures were incubated at 30 • C. Samples of 1 mL were collected at zero time, after 30 min, and after 120 min. The collected samples were immediately de-proteinized by centrifugation (4 • C, 5000× g, 10 min) in Amicon ® Ultra-4 Centrifugal Filter Units, 3 kDa cutoff limit. Samples of 50 µL were collected from the filtrates and subjected to HPLC-MS analysis.