The Publications Files/27 Photopharmacology

Minireviews ChemPhotoChem doi.org/10.1002/cptc.202100001

1 Very Important Paper 2 3 The Issue of Tissue: Approaches and Challenges to the 4 5 Light Control of Drug Activity 6 [a] [a] 7 Mayank Sharma and Simon H. Friedman* 8 9 10 Many of the major challenges associated with drug delivery can modulate drug release. Because of these and other advantages, 11 potentially be addressed by linking drug action to light a range of mechanisms for using light to manipulate drug 12 irradiation. These challenges include the spacing, timing and activity has been developed, including photocleavage control, 13 amount of a drug’s activity. Once a drug’s activity is linked to photoconformational control, photothermal control and photo- 14 light, this activity can be more easily manipulated, because light degradation control. These major themes of light control will be 15 itself is easy to manipulate. One of the main issues that light described in this minireview, and illustrated with examples. In 16 control can address is off-target toxicity. This has the potential addition, the issue of tissue light permittivity, arguably the 17 to be limited if drugs are activated only in target tissues using major challenge for the discipline, will be described and 18 light. For drugs that are needed at varying concentrations analyzed. 19 through the day, varying light has the potential to temporally 20 21 22 1. Introduction larger community of researchers using light to manipulate 23 biological phenomena. 24 Light is a powerful chemical reagent: Where light goes, when it 25 is applied, and the amount of light delivered are all factors that 26 are relatively easy to regulate. Because of this, there has been a 2. The Dimensions that Light Can Control 27 significant interest in controlling biological phenomena using 28 light, since so much of biology is linked to the concentration of There are three principal aspects of a process that light can 29 key molecules, as well as the timing and location of their modulate: a) Its spacing, b) its timing and c) the degree to 30 appearance. The areas of study influenced by these factors are which it happens. This is because once a process is linked to 31 numerous and include gene expression, developmental biology light irradiation, you can control where light goes, when light 32 and neuroscience. The molecular tools developed for these irradiation is initiated and how much light is applied. Each of 33 studies have been wide ranging, and include light-activated these dimensions of control confers different potential advan- 34 nucleic acids, proteins and small molecules.[1] Within this larger tages. 35 category of light controlled biology has been the development The control of spacing has the potential to reduce a drug’s 36 of light controlled drug activity.[1b,k,2] The motivation behind toxicity, by limiting its activity to a specific site, for example a 37 light controlled drug activity has been similar to that of light tumor or a site of infection. Many chemotherapeutics have 38 controlled biology in general: There are multiple drug classes dose-limiting toxicities associated with healthy tissue, for 39 for which control of the timing, amount and location of activity example non-cancerous but rapidly dividing cells. Limiting a 40 is critical for optimal treatment of disease. drug’s activity only to target sites where they are activated by 41 This review will focus on the literature that deals with light light could allow them to be used for longer periods of time or 42 control of drugs, approved molecules that can be used to treat at higher doses. 43 human diseases. This is as opposed to the larger (and The control of the timing and amount of release can be 44 important) literature that deals with light control of biological useful for drugs where the required amount varies continuously 45 probes, used typically to understand biological phenomenon. throughout the day. The majority of drugs likely do not fall into 46 We have attempted to identify the broad themes in the this category, as all that is required for their effective 47 literature, and illustrated them with representative examples. In application is that the systemic concentration remains above a 48 addition we have highlighted the strengths and weaknesses of critical therapeutic threshold. Normally this can be achieved 49 the approaches. Needless to say, many of the issues that are through more conventional means, such as extended release 50 involved in the light control of drug release also apply to the formulations.[3] However, there is a subset of applications where 51 timing and amount is critical, such as with hormones and other 52 [a] M. Sharma, Prof. S. H. Friedman signaling molecules where the requirements vary continuously 53 Division of Pharmacology and Pharmaceutical Sciences throughout the day. Light in this context has the potential to 54 School of Pharmacy confer a much needed level of timing and degree control. University of Missouri-Kansas City 55 Kansas City, MO 64108 (USA) 56 E-mail: [email protected] 57 An invited contribution to a Special Collection on Photopharmacology

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1 3. The Ways In Which Light Has Been Used To control of the activity of a biomolecule is the caging of ATP by 2 Control Drug Activity Hoffman and co-workers using an ortho-nitro benzyl group 3 that was capable of blocking the activity of ATP until 365 nm 4 irradiation released native ATP.[6] In the subsequent years, many 5 We can classify light controlled drug activity into four broad actual drugs have been modified with photocleavable (PC) 6 categories, which are linked to the mechanism of light control. groups and showed modulation of activity, although primarily 7 These are 1) Photocleavage control 2) Photoconformational in in-vitro settings. 8 control 3) Photothermal control and 4) Photodegradation The two photocleavable groups that have been most 9 control (Figure 1). With all four of these mechanisms, there are investigated for light activated drug control are the ortho-nitro 10 two principle characteristics of the photoactivated group that benzylic derivatives[7] and coumarin derivatives,[8] although a 11 determine the ultimate performance: The wavelength required wide range of other PC groups have also been explored. The 12 of the photoactivation, and the quantum yield for this process. ortho-nitro benzylic derivatives have absorption maxima in the 13 The wavelength strongly influences the depth that light can far UV (~365 nm). This is not an ionizing wavelength of UV 14 penetrate through tissue, with UV and short visible wave- light, but being shorter than visible light still suffers from lower 15 lengths penetrating a millimeter or less, and infrared having tissue penetration. For example, Nishimoto and co-workers 16 the potential to penetrate on the order of centimeters.[4] This used an ortho-nitro benzylic PC group to act as a linker 17 determines the ultimate number of photons that can reach the between the drug, 5 fluoro uracil, and a cyclic peptide targeting 18 desired target and photoactivated group, based on the amount moiety.[9] Lin and co-workers linked a porphyrin, itself capable 19 of light the body is irradiated with. In addition to wavelength, a of potential photodynamic therapy to 5 fluoro-uracil, via a light 20 critical descriptor of the photoactivated group is the quantum cleaved ortho-nitro benzylic group, showing light dependent 21 yield, the ratio of photostimulated events (e.g. photolysis) per cell toxicity.[10] The coumarin system has also been extensively 22 photon absorbed by the group. The ultimate nature of the examined, in studies of light activated drug and biological 23 photoactivation mechanism also influences the specific photo- probe release. This has resulted in the synthesis of derivatives 24 activated group that can be used. The photochemistry of with increasingly higher wavelengths of deprotection that now 25 specific photoactivatable groups has been recently described stretch to 500 nm and above.[8a,c,e] For example Feringa and co- 26 and reviewed in detail.[1k,2c,5] workers used two different coumarins with different depro- 27 Photocleavage: In photocleavage, irradiation is accompa- tection wavelengths to inhibit two separate strains of 28 nied by the breaking of a covalent bond between the drug and bacteria.[11] This was accomplished through the caging of two 29 a moiety that is responsible for modulating the activity of the different classes of antibiotics, a penicillin and a fluoroquino- 30 drug. Although not a drug, the earliest example of photo- lone. The coumarin system has also shown the potential for 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Figure 1. Four main approaches to the light control of drug action.

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1 radical recombination, resulting in undesired products. This is The rationale behind this approach is to switch the drug from 2 dependent on the nature of the linker. Multiple groups are an inactive state, unable to bind to the target of interest, to an 3 examining approaches to avoid this problem, including spacers active state after light is applied. The purpose of this, like much 4 to prevent unproductive recombination events.[12] of photocleavage activation, is to spatially trigger activity only 5 Outside of these two main PC groups, there have been in the target tissue, for example a site of bacterial infection or 6 multiple other photocleavable groups explored. For example, cancer metastasis. This can potentially limit toxicity in 7 Dore and co-workers have extensively explored the quinoline surrounding tissues, as well as in the environment. Often, with 8 based groups, applying them to the release of the drug a photoinduced conformational change there is a subsequent 9 tamoxifen.[13] McCoy and co-workers used the dimethoxy reversion to the initial conformational state, typically thermally 10 benzoin PC group to mask a range of carboxyl containing non initiated. This is a potential advantage of this approach over 11 steroidal anti-inflammatory drugs including ketoprofen and photocleavage, in that activated drug molecules that have 12 ibuprofen.[14] In addition, multiple examples of BODIPY modi- been activated in the target tissue and then diffuse away can 13 fied drugs have extended photocleavage well into the visible spontaneously revert to their inactive, and potentially non-toxic 14 range. For example Chakrapani and co-workers used an forms. 15 aryloxy-BODIPY derivative to modify a fluoroquinolone anti- For example, Feringa and co-workers made multiple 16 biotic, and then stimulate its release using 470 nm visible versions of the known proteasome inhibitor bortezomib, by 17 light.[15] In very promising work, Schnermann and co-workers grafting azo benzene moieties onto it. Several of these showed 18 have explored the cyanine chromophore for photocaging, and promise by having preferential inhibition of the target in the 19 showed it capable of being activated at an unusually high photoactivated conformation.[20] In a rare in-vivo demonstra- 20 wavelength (690 nm) through a two step process of photo- tion, the multi-disciplinary team of Trauner, Rutter, Hodson and 21 oxidation and hydrolysis. This represents the highest wave- co-workers used the azo benzene approach to control the 22 length single photon photocleavage process.[16] The use of light activity of a third generation sulfonylurea (JB253) to regulate 23 activated reactive oxygen species has also been used by several blood glucose, in-vivo, through the use of a fiber optic 24 groups to efficiently photocleave thioethers and alkenes.[17] delivered light.[21] This stimulated the formation of the active 25 In much of this work, the focus is chemical, demonstrating conformation of the sulfonylurea, with commensurate blood 26 that the PC group can be installed, and that light will release glucose reduction. The use of a fiber optic to irradiate the 27 the native drug. In addition, model systems have typically been pancreas may prove to be impractical for routine human 28 used to demonstrate activity, such as enzyme activity or a application. In another compelling example, Peifer and co- 29 specific bioactivity monitored in cell culture studies. What is workers demonstrated the possibility of a photoconformational 30 often missing from the work is a description of how the control of activity in an existing drug, the tyrosine kinase 31 resultant molecules would be used in an actual in-vivo setting, inhibitor axitinib.[22] 32 for example to treat human disease. One potential application A challenge that exists with the photoconformational 33 is to block activity until irradiation releases active drug in the approach is the difficulty of identifying molecules that have 34 desired tissue, thus avoiding the toxicity associated with strongly different inhibition profiles in their two different 35 systemic exposure. One challenge is that upon uncaging of the conformations. Often, inhibition of the light induced conformer 36 active drug in the target tissue, diffusion away from the site is is similar to that of the thermally more stable conformer. 37 possible. This has the potential to result in drug-induced Inhibition of the target can also be stronger with the “wrong” 38 toxicity in the surrounding tissue. i.e. thermally stable conformation. Significant iterations have 39 Our lab has used the photocleavage approach to control been pursued recently, with some examples showing signifi- 40 insulin release from a shallow, light accessible dermal depot. cant preference of the light induced conformer for the target.[23] 41 We accomplish this by linking photocleavage to a change in Other improvements include the increase of the photoisomeri- 42 solubility of the drug, in our case insulin. These approaches for zation wavelength to increase the tissue penetration ability of 43 solubility modulation include linking insulin to a polymer, the applied light. The overall challenge of tissue light 44 linking insulin to itself in a “macropolymer”, linking insulin to penetration will be discussed in greater detail later in this 45 highly non-polar moieties, and linking insulin to iso-electric review. 46 point shifting groups.[18] In all these cases, the result is a species Photothermal Control: One of the challenges of light 47 that is insoluble and able to form a depot but upon irradiation controlled drug release is the difficulty of delivering significant 48 will release native soluble insulin. The rationale behind such numbers of photons to the target site. With photochemical 49 materials is to use them in shallow, dermal depots that can be methods such as photocleavage, one is limited by the inherent 50 stimulated by surface irradiation to release insulin in response photochemistry: high energy processes such as bond breaking 51 to blood glucose information. typically require high energy photons to accomplish. These in 52 Photoconformational Control: Multiple groups are pursuing turn are associated with shorter wavelengths that have poor 53 the light control of drug action by incorporating photoinduced tissue penetration. An alternative to this is to use photons in an 54 conformational changes into drugs.[1k,19] A common motif to indirect way, as a source of localized heat that then stimulates 55 introduce this control is the azobenzene linkage, that can a physical change in a material that then allows drugs to be 56 switch from the typically thermodynamically more stable trans released. The photons utilized in such applications can be in 57 conformation to the cis conformation upon application of light.

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1 the infrared range and therefore capable of significant tissue hemolytic action of another peptide, melittin that is also bound 2 penetration. to the RBC via a lipid anchor. The blocking segment is bound 3 Gold nanoparticles are a common motif used to absorb to a porphyrin ring, via interaction with a cobalt atom, in a light 4 light and convert it to heat. For example Halas and co-workers cleaved link. Upon irradiation, the blocking segment is released, 5 demonstrated release of anti-cancer compounds such as taxol and the associated melittin is unmasked. Melittin can then 6 from gold shell SiO2 nanoparticles, using near IR radiation.[24] induce hemolysis of the RBC, releasing the therapeutic contents 7 The nanoparticles were modified with albumin that bound of the cell. A significant strength of this approach is that the 8 drug, until irradiation denatured the protein and released active activation wavelength is as high as 660 nm.[29] 9 drug. They were able to demonstrate cellular uptake of the A significant advantage of the photodegradation approach 10 nanocomplexes as well as a light dependent toxicity towards is the potential, at least in theory, to have more than one drug 11 cells in culture. In another example, Xia and co-workers used molecule released for each photo-stimulated event. In photo- 12 gold “nanocages” to control the release of doxorubicin using cleavage or photoconformational control, each photostimu- 13 near-IR laser irradiation.[25] In this example, heat generated by lated event is associated with a single drug being released or 14 the interaction of light with the gold nanocages triggered a activated. This potential for amplification is expected to be 15 thermally-induced conformational change in polymers adhered dependent on the nature of the drug entrapment. In addition, 16 to the cages. This then allowed entrapped drug to be released. there is the potential that one material can be applied to any 17 The effectiveness was demonstrated with both small molecules therapeutic, once it has been optimized. A potential disadvant- 18 and proteins (the model lysozyme). Zhou and co-workers used age is the unknown systemic effects of the polymers/oligomer 19 a gold/silver hybrid nanoparticle system, to control the release left behind after photodegradation. 20 of a model (curcumin) using near-IR irradiation. Curcumin A field with significant overlap with the light activation of 21 release was triggered by the temperature responsive swelling/ drug activity and all the mechanisms described above, is 22 deswelling transition of an outer PEG layer.[26] A potentially photodynamic therapy.[30] In both, light is used to activate a 23 significant challenge with the photothermal approach is the molecule to achieve a desired therapeutic outcome. In photo- 24 cost and unknown toxicity associated with long term exposure dynamic therapy (PDT), this activation is mediated via a 25 to metal nanoparticles. photosensitizer that leads to the formation of reactive oxygen 26 Photodegradation Control: In this mechanism, photocleav- species, that are then able to induce cytotoxicity at the site of 27 able groups are used to link oligomers and in so doing create irradiation. Thusly, PDT application is limited to disease states 28 larger species, such as polymers or micelles, that can entrap that benefit from the induction of cytotoxicity, such as cancer 29 drug molecules non-covalently. Upon irradiation the links and age-related macular degeneration, whereas the photo- 30 between oligomers can be photocleaved, changing the activation of drug activity has a greater potential breadth of 31 structure of the species (e.g. increasing porosity) allowing therapeutic application, namely any disease state that can be 32 entrapped drug molecules to be released. Almutairi and co- treated with a drug. PDT has certain advantages however, 33 workers applied the photodegradation approach using a new specifically linked to the photochemistry of the photosensitizer. 34 light sensitive polymer that contains multiple quinone methide The engineering of high wavelength, and importantly high 35 self-immolative moieties installed along the backbone. Upon quantum efficiency photosensitizers has proven to be less 36 irradiation it showed burst release of a model small molecule challenging than doing the same for photocleavable groups. 37 dye, a proxy for a drug.[27] Gillies and co-workers used a related Ultimately however, PDT faces the same challenge in terms of 38 approach for the light stimulated release of paclitaxel. They light penetration, and its applications have been limited largely 39 incorporated a light cleavable ortho nitro benzylic crosslinking to surface or near surface applications (e.g. skin cancer, macular 40 group into a poly (ester amide). This when formulated with degeneration), or sites that are accessible to fiber guided light 41 poly(ethylene oxide) and paclitaxel produced a micellar materi- (e.g. lung cancer, bladder cancer). 42 al that responded to 365 nm UV light. The light activated 43 crosslinks were cleaved, destabilizing the micelles, and releas- 44 ing paclitaxel.[28] Using a similar ortho nitro benzyl based 4. The Issue of Tissue Light Penetration 45 crosslinker, Tong and co-workers created nanocapsules of 46 polyethyleneimine (PEI). Amine groups on PEI were both Independent of which mechanism is being used, the promise 47 alkylated and acylated using the crosslinker 4-bromomethyl-3- of light activated drug action hinges on light reaching the 48 nitrobenzoic acid. The resulting crosslinked PEI was fashioned target site in an amount sufficient to release a therapeutically 49 into “nanocapsules” that contained a fluorescently labeled relevant amount of drug. This is a major and possibly 50 dextran as a proxy for a drug payload. Upon photolysis, the insurmountable problem in the most general sense. It hinges 51 crosslink was broken, allowing the contents of the nanocapsule on many variables: the concentration of drug required for 52 to be released. therapeutic action, the pharmacokinetics of the drug (i.e. rate 53 Lawrence and co-workers have used a type of photo- of uptake/clearance and the likelihood of the target concen- 54 degradation to create an engineered red blood cell (RBC) that tration being achieved), the differential efficacy found between 55 releases therapeutic proteins in response to light. They different photo states (for photo switched as opposed to 56 accomplished this by introducing a light activated blocking photoreleased drugs), the location of the site of action, the 57 segment (BS) to the surface of the RBC that blocks the location of the site of irradiation, the light absorptivity of the

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1 drug, the quantum yield for turning that absorbed light into environment. These are significant and reasonable goals. Two 2 the desired activation, and finally the amount of light reaching major stated applications are cancer and bacterial infection. 3 the site of irradiation. Most of these factors have the potential The question we now ask is what is the ultimate limit for 4 to be optimized, by careful design and iteration by the chemist. non-surface applications of light activated drug release? Is it a 5 The last factor however is the most challenging as it is bound reasonable goal or does it lie outside of the realm of possibility 6 by physical and biological limitations: the inherent amount of given the physical constraints of the system? To begin to 7 light that can penetrate tissue to reach the activation site answer this question we will examine the parameters for a 8 without inducing thermal damage to the intervening tissues. hypothetical target, and predict the total energy needed to 9 In the earliest days of this discipline, the issue of tissue light produce the desired therapeutic effect. The ultimate constraint 10 penetration was largely ignored, while the fundamental is the amount of heat generated in tissue through the process 11 concepts and approaches for light activated drug release were of irradiation. There is a limit to how much heat can be 12 explored. These early studies focused on the synthesis of dissipated from tissues, after which irreversible thermal damage 13 modified drug species whose activity could be toggled with will be the result. Our approach for this analysis is to start at 14 light and then followed up with a demonstration of in-vitro the target site and then conceptually work our way back out to 15 release of the drug. The selected photoswitchable or photo- the body surface, to ultimately determine the required amount 16 cleavable groups often used short wavelengths, resulting in of surface applied photons to achieve the desired therapeutic 17 systems that would likely not be effective in animals. The effect. We will make our assumptions explicit and attempt to 18 kinetics of drug release were carefully monitored but in in-vitro make them conservatively so as to define a limiting case. 19 settings that allowed for convenient control of important We consider a spherical target site with a radius of 1 cm, 20 factors, such as light intensity. We can see this work as the though clearly some targets can be smaller or larger. This is a 21 necessary early steps in the development of the discipline. If reasonable value for a “typical” treatment site for a tumor or 22 these steps fail then there is no need to try the later and more bacterial infection site. This results in a target volume of 4.2× 23 challenging steps (e.g. in-vivo activity). New iterations have 10À 3 liters. We place this target at a depth of 5 cm from the 24 attempted to push the field in the “right direction” i.e. through skin’s surface. This is clearly not as deep a target as is possible 25 the incorporation of higher and higher wavelength photo- in the human body, but is a reasonable representative of a 26 cleavable/switchable groups that should allow easier access by “non-surface” target. In this volume we require active drug. A

27 light. As the field has matured, and the number of examples of good drug will have a Kd value for its target of 1 nM and if we 28 photo released drugs has increased, and as multiple iterations wish for a majority of the target to be bound, a concentration 29 have solved early challenges, it seems fair to examine closely that is minimally ten times this is reasonable or 10 nM of active 30 the issue of the penetration of light through tissue to better drug in the target volume. This then corresponds to 4.2× 31 define the limits of this discipline and to also determine the 10À 11 moles. 32 disease states and applications that would be best suited for a Photons arriving at the target tissue first have to be 33 photo activated drug approach. absorbed by the drug, and then converted into active drug by 34 Recently, Feringa, Szymanski and co-workers made an initial a photochemical process. The proportion of target-applied 35 critical analysis in this direction, breaking down potential photons that are absorbed by the drug is linked to the 36 medical applications of light activated drugs based on the ease concentration of the drug, the extinction coefficient of the 37 of light access to the afflicted tissue.[19] These range from the drug and the pathlength. We have already determined that we 38 skin and eye (i.e. directly accessible via light without significant need 1×10À 8 M minimum of drug concentration. The path- 39 intervention), to the interior of the body (which is much more length will vary over the cross-section of the spherical target, 40 difficult to illuminate). Sites of intermediate potential include but we will assume a 2 cm distance, the largest diameter of the 41 targets that are accessible endoscopically. Our lab’s work on target. Previously described light activated drug candidates 42 the photoactivated depot approach largely side steps these have varying extinction coefficients, so we use a generous 43 issues, as it designed around a shallow, skin based depot in value of 10,000 MÀ 1cmÀ 1, understanding that most will be lower 44 which drug is contained, and then released by trans-cutaneous than this, and less light will be absorbed. Given these 45 irradiation. The amount of tissue that needs to be traversed by parameters of 1×10À 8 M drug concentration, 2 cm pathlength 46 light is inherently on the order of millimeters. What is lost in and 10,000 MÀ 1cmÀ 1 extinction, we calculate an absorbance of 47 the photoactivated depot approach is spatial targeting. For the drug in the target tissue of 2×10À 4, although with many 48 some applications, for example insulin release, spatial targeting photo-drugs this absorption could be an order of magnitude 49 is less important than the timing and amount of drug release, less. We are not considering photon losses within the target 50 both of which can be controlled by light being directed at the due to scattering or tissue absorption in the target, though this 51 known site of depot injection. But for many applications, the too will reduce the number of photons able to be absorbed by 52 long stated goal of photopharmacology has been to spatially the drug. We will explicitly deal with the light attenuation due 53 target tissues of the body for drug activation, thus producing to tissue between the skin and target shortly. 54 active drug only at the target site, and therefore reducing the Given the calculated absorption value of the drug in the 55 overall toxic burden to the healthy tissues of the body, or target tissue, what proportion of photons arriving at the target

56 reducing the toxic burden of the drug molecules on the tissue will be absorbed by the drug? This proportion is (I0-I)/I0,

57 where I0 is the intensity of the light arriving at the target, and I

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1 is the intensity of the light departing the target after absorption From the previous calculation, we determined that we À 11 2 has taken place. This corresponds to 1-I/I0. Given that the needed 4.2×10 moles of active drug in the target tissue. This À A 13 À 10 3 absorbance A=log (I0/I), and I/I0 =10 then the value of 1-I/I0 corresponds to 2.5×10 molecules. Given that 4.6×10 of the 4 evaluates to 1–10À A. This results in a proportion of the light skin applied photons get converted into active drug, this 5 arriving at the target that is absorbed by drug as 4.6×10À 4. The indicates that we need to apply 2.5×1013/4.6×10À 10 photons or 6 amount of drug converted by light into its active state is 5.4×1022 photons to the skin to get the required drug release. 7 related to this amount of light absorbed by the target and the A typical infrared photon of 700 nm carries 2.84×10À 19 J of 8 quantum yield of the photochemical process. The quantum energy. Thus the total amount of energy applied to the skin is 9 yield too is a highly variable number. We have assigned a 5.4×1022 ×2.84×10À 19 =1.54×104 J. Only a tiny proportion of 10 quantum yield of 0.1 to our hypothetical system which is this total energy results in drug activation and the majority of 11 arguably generous. Given this quantum yield, the proportion of this energy will be converted to heat in the tissue, through 12 light arriving at the target that is absorbed by the drug and both absorptive and internal scattering processes. The 1 cm 13 then converted into active drug is 4.6×10À 5. radius, 5 cm high cylinder of tissue between the skin and the 14 We will now consider the amount of light that reaches the target has a volume of 5×π or 15.7cc. 1.54×104 J is about four 15 target in the first place, after having been applied to the skin times the energy required to boil the equivalent amount of 16 and traversing 5 cm of tissue (Figure 2). The light attenuation of water in the volume between the skin and the target tissue. 17 tissue has been extensively studied, in varying tissues and with Needless to say irreversible tissue damage would occur at even 18 varying wavelengths. The existence of a “window” in the infra- a fraction of the amount of energy required to achieve a 19 red region has been well described. This does not mean therapeutic effect. This effect is intensified because a majority 20 however that tissue is transparent in this spectral region, just (~90%) of the light is absorbed (and hence heat generated) in 21 that absorbance is at a relative minimum. Bashkatov and co- the outermost 1 cm of tissue which will take the majority of the 22 workers have extensively studied light transmittance in a wide thermal damage. 23 range of tissues, including skin, colon, and others.[4c] In There are multiple assumptions made in this calculation, 24 reviewing the absorption coefficients in this IR window of 600– but we have attempted to be conservative, to define a limiting 25 1300 nm, they indicated that the typical value was 2.5� case. One parameter that can be varied is the depth, as it is 26 1.5 cmÀ 1.[4a] This is a value that is consistent with other group’s associated with an ~10-fold change in intensity with each cm. 27 analyses of light penetration in tissues.[31] Based on this we Shallower targets, as has been previously suggested, will likely 28 used a conservative value of 1 cmÀ 1, although some tissues have a better chance at success. There may be ways of 29 such as skin appear to be significantly higher, an observation mitigating tissue thermal damage by spreading the light “dose” 30 confirmed by multiple authors. This is an absorbance coef- out over time, or through the equivalent of tomography, 31 ficient, so a value of 1 cmÀ 1 corresponds to an attenuation wherein multiple converging beams reduce the overall thermal 32 factor of 10 for each cm of tissue traversed. At a depth of 5 cm, burden. But given the apparently very high thermal burden of 33 this is a proportion of 10À 5 of the skin applied light arriving at whole body applications of photopharmacology that this 34 the target. This, combined with the factor of 4.6×10À 5 for the analysis indicates, it seems reasonable to attempt to explore 35 proportion of light arriving at the target that is converted to the ways in which this burden can be alleviated or indeed if it 36 active drug, means that a total of 4.6×10À 10 of the skin applied can be alleviated in the most general sense. 37 photons get converted into active drug in the target site. 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Figure 2. Analysis of active drug formation versus surface applied photons.

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Wiley VCH Mittwoch, 10.03.2021 2199 / 197084 [S. 8/9] 1 1 MINIREVIEWS 2 3 M. Sharma, Prof. S. H. Friedman* 4 5 1 – 9 6 7 The Issue of Tissue: Approaches 8 and Challenges to the Light 9 Control of Drug Activity 10 Drug activity can be effectively promising as these approaches are, 11 modulated with light using a wide the attenuation of light and conver- 12 range of mechanisms, including pho- sion to heat in practice pose signifi- 13 tocleavage control, photoconforma- cant barriers to application to non- 14 tional control, photothermal control surface sites. 15 and photodegradation control. As 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

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