**6. Effect of covalent drugs conjugation on the pharmacokinetic profile of the drug**

The pharmacokinetic profile of drugs is a very crucial aspect that is considered for clinical application. It is interesting to observe that the pharmacokinetic profile of drugs is becoming better due to covalent drug conjugation as demonstrated by drugs encapsulated to nano/micro-delivery systems. The conjugation of drugs to biomaterials has opened opportunities to alter the pharmacokinetics and biodistribution of the drugs within the human body [120]. The alteration of the pharmacokinetics of the drug offers advantages such as prevention of the rapid clearance or metabolism of the drug. In addition, the drugs are carried to the targeted site of pharmacological action. Drug conjugation to nano/micro-delivery system has shown to be a powerful technique that can alter the pharmacokinetic profile of the drug, thus minimize the side effect of various anticancer drugs such as doxorubicin.

One of the studies that reported the enhanced pharmacokinetic properties of anticancer drug encapsulated to a nanoparticle is that conducted by Vandriess et al. [80]. It was observed that the nanoparticles containing the drug enhanced the drug accumulation and a subsequent reduction of tumor growth in an *in vivo* zebrafish model. Also, another study has revealed that self-assembling drug polymer conjugates which allows a covalent attachment of the drug to the hydrophilic part of the polymer can improve the pharmacokinetic profile of drugs. Thus, covalently

conjugating drugs to delivery systems improves pharmacokinetic profiles and physicochemical problems associated with certain free drugs [142]. Benefits from covalent conjugations include prevention of rapid renal clearance, and improved drug solubility is derived. It is however important to note that the attachment of a large number of hydrophobic drugs to nanocarriers such as polymers may result in unwanted aggregation and precipitation in some cases [143].

Similarly, more recent studies have further shown that covalent linked prodrugs

The delivery of drugs to sites of target, where the action of the drug is required, is challenging due to various physicochemical, biopharmaceutical, and pharmacokinetic barriers the drug may face [143]. In order to address these issues, new approaches such as drug covalently conjugated to nano/micro-delivery systems are being explored which alters the pharmacokinetic properties of the drug. This is achieved by using drug various carriers such as polymers [148, 149] liposomes [150], and dendrimers [151] that are capable of protecting the payload drug and delivering it to the disease site. The nano/micro-delivery systems help in accumulating the drug in the tumor site and prolong the circulation time [80]. This consequently leads to a successful drug target to the specific disease site. Drug conjugates have been widely utilized in the field of cancer therapy because they can passively target cancer disease sites by permeating and retain the drug via tumor's leaky vasculature [152]. Apart from the ability of the nano/micro-delivery system to protect the drug from degradative processes such as hydrolysis and metabolism before arriving at the target site, the drug is able to accumulate in the targeted site. This ability is a major advantage that drug conjugates proffers, which enhances their antitumor activity. Additionally, the ability of the drug conjugate to disassemble provides the opportunity to tune the drug release rate at the target site. More so, the tuning or decorating the surface of the nanostructure (drug conjugate) can lead to enhanced tumor targeting compared to drugs in their free form [143]. The enhanced drug efficacy reported in the studies discussed earlier is closely associated

with the ability of the drug conjugate to target the specific disease site.

**155**

The advantages of covalently conjugating drugs to nano/micro-delivery systems to target disease sites or site of infections has shown effective by results obtained by various researchers who employed different therapeutic agents and nano/microdelivery systems in targeting specific disease sites as highlighted in **Table 3**. In all the reported study on drug targeting using drug conjugates, a superior targeting of drugs to disease site was displayed by the drug encapsulated to nano/micro-delivery

improve the pharmacokinetics and biodistribution of the conjugated drug as reported by Ibrahim and co-workers. H22 tumor model was induced in BALB/c [115]; the mice were exposed to conjugated drug delivery system and free-drug camptothecin (CPT), respectively. From the results, conjugates had a significantly had better pharmacokinetic profile than the free drug, this also led to increased accumulation of the drug within the tumor tissues and consequently better activity. This observation can be associated with the enhanced permeability and retention (EPR) effect provided by the nano/micro-delivery system. An additional observation which is the biodistribution effect of the drug conjugate is seen in the outstanding inhibition of tumor growth that resulted in better tumor shrinkage when compared to other groups (**Figure 10**). After 21 days of treatment, the drugconjugated delivery system had a 33-fold less of tumor when compared to the untreated group. It is presumed that the acidity of the tumor contributed to the enhanced anticancer effect by the drug conjugated delivery system after cellular uptake of the nanoparticles, and the drug was released in the cytosol. This might have been the reason for the overall better activity of the conjugated drugs compared to the free drug. This was attributed to the extended blood circulation and better accumulation of conjugates in tumor compared to free CPT. Overall, these studies suggest that by covalently conjugating drugs into nano/micro-delivery systems, and a more enhanced pharmacokinetic profile of drugs can be obtained.

*Nano/Microparticles Encapsulation Via Covalent Drug Conjugation*

*DOI: http://dx.doi.org/10.5772/intechopen.93364*

**7. Disease site targeting via covalent conjugation**

In one of the first studies, research groups such as Kataoka et al. took advantage of the self-aggregation behavior of drug conjugates to develop a micelle forming drug conjugate [144]. In their study, doxorubicin (DOX) was conjugated to a poly (ethylene glycol)-poly(aspartic acid) block copolymer (PEG-b-P(Asp(DOX)), and the pharmacokinetic profile was investigated. It was observed that there was no interaction between the drug and serum albumin, which is known to bind to the DOX. The inability for the drug and the serum albumin to interact indicates the shielding ability of the nano/micro-delivery system which led to a good biodistribution and therapeutic effect of the drug. Other subsequent studies by the same research group aimed to improve the synthesis [145] and pharmacokinetics profile [146, 147] of the same drug-polymer conjugate. The drug conjugate was designed via an amide bond which can only cleave to release the drug by enzymatic action. The results showed that the drug conjugate had a better pharmacokinetic profile when compared to the free DOX. Interestingly, the conjugate was better tolerated despite needing a higher dose to achieve the same effect as free DOX.

#### **Figure 10.**

*Control (phosphate buffer system), camptothecin (CPT), and micellar nanoparticles drug conjugates (PC-NPs and PCI-NPs) effectiveness on 22 tumor-bearing BALB/c mice after (A) graphs displaying tumor growth inhibition (B) effect on tumor shrinkage after treatment with PBS, CPT, PC-NPs, and PCI-NPs at 21st day post-treatment of the mice (C) micro-photographs of the harvested tumors. (D) Changes in the bodyweight of the mice during treatment. (E) H&E histological images of different treatment groups. Adapted with permission from [115].*

#### *Nano/Microparticles Encapsulation Via Covalent Drug Conjugation DOI: http://dx.doi.org/10.5772/intechopen.93364*

conjugating drugs to delivery systems improves pharmacokinetic profiles and physicochemical problems associated with certain free drugs [142]. Benefits from covalent conjugations include prevention of rapid renal clearance, and improved drug solubility is derived. It is however important to note that the attachment of a large number of hydrophobic drugs to nanocarriers such as polymers may result in

In one of the first studies, research groups such as Kataoka et al. took advantage of the self-aggregation behavior of drug conjugates to develop a micelle forming drug conjugate [144]. In their study, doxorubicin (DOX) was conjugated to a poly (ethylene glycol)-poly(aspartic acid) block copolymer (PEG-b-P(Asp(DOX)), and the pharmacokinetic profile was investigated. It was observed that there was no interaction between the drug and serum albumin, which is known to bind to the DOX. The inability for the drug and the serum albumin to interact indicates the

unwanted aggregation and precipitation in some cases [143].

*Nano- and Microencapsulation - Techniques and Applications*

**Figure 10.**

*from [115].*

**154**

shielding ability of the nano/micro-delivery system which led to a good

biodistribution and therapeutic effect of the drug. Other subsequent studies by the same research group aimed to improve the synthesis [145] and pharmacokinetics profile [146, 147] of the same drug-polymer conjugate. The drug conjugate was designed via an amide bond which can only cleave to release the drug by enzymatic action. The results showed that the drug conjugate had a better pharmacokinetic profile when compared to the free DOX. Interestingly, the conjugate was better tolerated despite needing a higher dose to achieve the same effect as free DOX.

*Control (phosphate buffer system), camptothecin (CPT), and micellar nanoparticles drug conjugates (PC-NPs and PCI-NPs) effectiveness on 22 tumor-bearing BALB/c mice after (A) graphs displaying tumor growth inhibition (B) effect on tumor shrinkage after treatment with PBS, CPT, PC-NPs, and PCI-NPs at 21st day post-treatment of the mice (C) micro-photographs of the harvested tumors. (D) Changes in the bodyweight of the mice during treatment. (E) H&E histological images of different treatment groups. Adapted with permission*

Similarly, more recent studies have further shown that covalent linked prodrugs improve the pharmacokinetics and biodistribution of the conjugated drug as reported by Ibrahim and co-workers. H22 tumor model was induced in BALB/c [115]; the mice were exposed to conjugated drug delivery system and free-drug camptothecin (CPT), respectively. From the results, conjugates had a significantly had better pharmacokinetic profile than the free drug, this also led to increased accumulation of the drug within the tumor tissues and consequently better activity. This observation can be associated with the enhanced permeability and retention (EPR) effect provided by the nano/micro-delivery system. An additional observation which is the biodistribution effect of the drug conjugate is seen in the outstanding inhibition of tumor growth that resulted in better tumor shrinkage when compared to other groups (**Figure 10**). After 21 days of treatment, the drugconjugated delivery system had a 33-fold less of tumor when compared to the untreated group. It is presumed that the acidity of the tumor contributed to the enhanced anticancer effect by the drug conjugated delivery system after cellular uptake of the nanoparticles, and the drug was released in the cytosol. This might have been the reason for the overall better activity of the conjugated drugs compared to the free drug. This was attributed to the extended blood circulation and better accumulation of conjugates in tumor compared to free CPT. Overall, these studies suggest that by covalently conjugating drugs into nano/micro-delivery systems, and a more enhanced pharmacokinetic profile of drugs can be obtained.
