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Tuesday, November 16, 2010

Herb–drug interactions: an overview of the clinical evidence

Herb–drug interactions: an overview of the
clinical evidence




ABSTRACT

Herbal medicines are mixtures of more than one active ingredient. The multitude of pharmacologically active compounds obviously increases the likelihood of interactions taking place. Hence, the likelihood of herb–drug interactions is theoretically higher than drug–drug interactions, if only because synthetic drugs usually contain single chemical entities. Case reports and clinical studies have highlighted the existence of a number of clinically important interactions, although cause-and-effect relationships have not always been established. Herbs and drugs may interact either pharmacokinetically or pharmacodynamically. Through induction of Cytochrome P-450 enzymes and/or P-glycoprotein, some herbal products (e.g. St John’s wort) have been shown to lower the plasma concentration (and/or the pharmacological effect) of a number of conventional drugs, including cyclosporine, indinavir, irinotecan, nevirapine, oral contraceptives and digoxin. The majority of such interactions involve medicines that require regular monitoring of blood levels. To date there is less evidence relating to the pharmacodynamic interaction. However, for many of the interactions discussed here, the understanding of the mechanisms involved is incomplete. Taking herbal agents may represent a potential risk to patients under conventional pharmacotherapy.

KEY words
 Herbal drugs, drug–drug interactions, Cytochrome P-450, oral contraceptives, indinavir,

















INTRODUCTION

Due to the growing use of herbals and other dietary supplements healthcare providers and consumers need to know whether problems might arise from using these preparations in combination with conventional drugs. The use of herbal products has dramatically increased over the past decade, driving physicians to become educated in regards to potential herbal complications and drug interactions. From 1990 to 1997, the herbal product market increased by 48%, with 42% of the population using alternative treatments and spending an estimated $27 billion on them [1] Herbal products are widely available, relatively inexpensive, and often make alluring but unsubstantiated claims. Herbal medicine appeals to consumers who believe that natural herbal products are preferable to synthetic pharmaceuticals [2]. A relevant safety concern associated to the use of herbal medicines is the risk of interaction with prescription medications [3-7]. This issue is especially important with respect to drugs with narrow therapeutic indexes, such as warfarin or digoxin [8]. Recent examinations have indicated that as many as16% of prescription drug users consume herbal supplements . Exacerbating the problem, herbal remedies are often marketed on the Internet with misleading and unproved claims.
Despite repeated warnings, consumers continue to equate ‘‘natural’’ with safe. As use becomes more prevalent and reports of adverse effects continue mount, there is an increasing need for health care professionals to understand better the potential complications associated with these herbal remedies. There are numerous products currently on the market that have been associated with toxicity. The aim of this article is to highlight the clinical interactions between herbal remedies and prescribed drugs. Theoretical herb drug interactions, which are based on in vitro experiments, animal studies, speculative and/or empirical evidence can be found elsewhere[9].


The nature of herbaldrug interactions
Natural products, unlike conventional drugs, provide a complex mixture of bioactive entities, which may or may not provide therapeutic activity. Often a complete characterization of all the chemical constituents from a natural product is unknown.
Additionally, chemical makeup of a natural product may vary depending on the part of the plant processed (stems, leaves, roots), seasonality and growing conditions. Combination products composed of multiple natural products complicate matters further. Not only does the complex nature of natural products complicate the determination of drug–herbal interactions, but the manufacturing process contributes to the overall complexity as well. Because herbal products are not regulated by the FDA, as previously stated, there are no standards for herbal products. Indeed, some products have been found to be misidentified, substituted and/or adulterated with other natural products or unwanted substances. Testing of the quality of more than 1200 dietary supplement products by the independent laboratory ConsumerLab.com found that 1 in 4 dietary supplement products lacked the labeled ingredients or had other serious problems such as unlisted ingredients or contaminants. This creates a problem when evaluating the validity of drug–herbal interactions [10].


MECHANISMS OF HERB–DRUG INTERACTIONS

Herbal medicines follow modern pharmacological principles. Hence, herb–drug interactions are based on the same pharmacokinetic and pharmacodynamic mechanisms as drug–drug interactions [11]. Pharmacokinetic interactions have been more extensively studied and in vitro and in vivo studies indicated that the altered drug concentrations by co-administered herbs may be attributable to the induction (or inhibition) of hepatic and intestinal drug-metabolizing enzymes [particularly Cytochrome P-450 (CYP)], and/or drug transporters such as P-glycoprotein [12-13]. The CYP is the most important phase I drug-metabolizing enzyme system, responsible for the metabolism of a variety of drugs. Many herbs (e.g. St John’s wort, echinacea, kava and garlic) and natural compounds isolated from herbs (e.g. flavonoids, coumarins, furanocoumarins, anthraquinones, caffeine and terpenes) have been identified as substrates, inhibitors and/or inducers of various CYP enzymes [14]. Specifically, clinical studies have shown that long-term (2 weeks) St John’s wort administration significantly induced intestinal and hepatic CYP3A4 and possibly other CYP enzymes involved in drugs metabolism [15-21]. Moreover, a clinical study performed on 12 healthy subjects showed that echinacea modulated the catalytic activity of CYP3A at hepatic and intestinal sites (induction of hepatic CYP3A4 and inhibition of intestinal CYP3A4). By contrast, a number of herbal medicines, including green tea ginkgo garlic  saw palmetto  and Siberian ginseng [22] did not affect CYP3A4 and CYP2D6 activities in normal volunteers. P-glycoprotein in the intestine, liver and kidney may play an important role in the absorption, distribution, or excretion of drugs. P-glycoprotein appears to limit the cellular transport from intestinal lumen into epithelial cells and also enhances the excretion of drugs out of hepatocytes and renal tubules into the adjacent luminal space [23]. Like CYP, P-glycoprotein is vulnerable to inhibition, activation, or induction by herbs and herbal constituents. Curcumin, ginsenosides, piperine, sylimarin and catechins may affect P-glycoprotein-mediated drug transport [24]. St John’s wort induces the intestinal expression of P-glycoprotein both in isolated cells and in healthy volunteers. Hyperforin, a major ingredient of St John’s wort, binds to orphan pregnane X receptor  resulting in a series of intracellular events leading to the expression of CYP3A4 and P-glycoprotein. A few pharmacodynamic interactions have also been described. Pharmacodynamic interactions may be additive (or synergetic), whereby the herbal medicine potentiates the action of synthetic drugs (e.g. interaction between the anticoagulant warfarin with antiplatelet herbs), or antagonistic, whereby the herbal medicine reduces the efficacy of synthetic drugs (e.g. kava possesses dopaminergic antagonistic properties and hence might reduce the pharmacological activity of the anti-parkinson drug levodopa) [25].


LIMITATIONS
Much of the available information about the interaction between herbal products and prescribed drugs is gleaned from case reports, although clinical studies are now also beginning to appear in the literature. The published case reports are often incomplete as they do not allow us to conclude that a causal relationship exists. Even documented case reports have to be interpreted with great caution, as causality is not usually established beyond reasonable doubt. According to the scoring system described by Fugh-Berman and Ernst [26], 68.5% of the cases reported were classified as ‘unevaluable’ (i.e. reports contained inadequate information to assess the likelihood of an interaction), 18.5% were classified as ‘possible’ (i.e. reports provided some evidence for an interaction, but there may be other causes of the event) and 13% as ‘well documented’ (reports appeared to provide reliable evidence for an interaction.

Herbal medicine involved in Drug interaction
Drug
Herb
Result of interaction
Possible mechanism
Source of evidence
Referene

Digoxin

Gum guar,
St John’s wort, wheat bran

Decreased plasma
digoxin concentration

Multiple mechanisms:
(i) gum guar delays gastric emptying and
hence may reduce digoxin absorption
(ii) St John’s wort induces
P-glycoprotein which is involved in digoxin
absorption/
excretion
(iii) Fibres in bran may trap digoxin in the gut.

Clinical studies

   27-30

Aspirin

Ginkgo

Spontaneous hyphema

Additive effect on platelet aggregation (ginkgolides have antiplatelet activity)

A case report

  31

Lovastatin

Pectin or oat bran

Decreased absorption
of lovastatin

Pectins or bran fibres may bind or trap lovastatin in the gut.

A clinical study

32
Simvastatin

St John’s wort

Decreased plasma
digoxin concentration

Simvastatin is a substrate of P-glycoprotein
and is metabolized by CYP enzymes.
Both CYP enzymes and P-glycoprotein are
induced by St John’s wort

A clinical study

33
Phenprocoumon

Ginger




St John’s wort

(i) Over-anticoagulation



(ii) decreased
anticoagulant effect

(i) Additive effect on coagulation (ginger
inhibits platelet aggregation)
(ii)Phenprocoum n is metabolised by cytochrome enzymes which are induced
by St John’s wort.

A case report




A clinical study

34




35
Verapamil

St John’s wort

Decreased bioavailability
of verapamil

Induction of intestinal CYP3A4 by
St John’s wort.

A clinical study

36
Alprazolam







Amitriptyline







Buspironea

St John’s wort







St John’s wort







St John’s wort

Decreased plasma levels of alprazolam





Decreased plasma levels of amitriptyline





Hypomania
Alprazolam is a specific probe for CYP 3A4,
which is induced by St John’s wort.



Amitriptyline is a substrate of both CYP2C19
and P-glycoprotein which are induced by St John’s wort.

Synergistic effect on 5-HT receptors
Clinical studies






A clinical study






A case report
37-38






 
39







40






Conclusion
Based on current evidence from in vitro, in vivo and clinical studies, herbals and other dietary supplements interact with many drugs. Still, drug-herbal interactions are difficult to evaluate because of the lack of reliability of these products. The interactions often involve drug-metabolizing enzymes and drug transporter systems, although pharmacodynamic interactions can also be involved. Because the pharmacokinetic and pharmacodynamic characteristics of most herbal and other dietary supplements are not completely recognized, potential interactions are not often predictable. Potential interactions are more likely to occur with drugs with narrow therapeutic indexes. The evidence-based evaluation used in the study can be used to evaluate the reliability of case reports.





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42 Schelosky L., Raffauf C., Jendroska K., Poewe W. Kava and dopamine antagonism. J. Neurol. Neurosurg. Psychiatry (1995) 58  639–640.


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44  Dandekar U.P., Chandra R.S., Dalvi S.S. et al. Analysis of a clinically important interaction between phenytoin and  Shankhapushpi, an Ayurvedic preparation. J. Ethnopharmacol. (1992) 35  285–288.

45  Shaw D., Leon C., Kolev S., Murray V. Traditional remedies and food supplements. A 5-year toxicological study (1991– 1995). Drug Saf. (1997) 17  342–356.




Monday, November 15, 2010

pharma job cover letter new

Your name

Your Address

Your City state, zip



Your Phone number,

Your Email

Date

Name

Position,

Department

Office address,

City, State, Zip.

Dear Sir,

This is in reference to the pharmacy job advertised in the (name of the newspaper) dated (provide the date).

I am a pharmacy graduate from ( university name ) and has done my post graduation from ( university name).I am currently working in( name of the company ) and is interested to move to your company as I have read that you work on the modern technology.

I feel that the job at your company is more challenging and I can secure more experience and knowledge working at (company name).I had always dreamt of working in leading pharmaceutical company like yours where I can get trainees in the modern medical sciences and grow professionally.

I am sure that the exposure and skills that I have gained from the current company will help me work in your office. As I am sure my qualifications and experience will match with your requirements and assure you that with my dedication and hard work we can get mutual benefits.

I have enclosed my resume for your review and I am looking forward to attend an interview and discuss further interest in joining your company.

Thanking you ,

Sincerely,

Your Name.

sample cover letter for pharma jobs

Pharma jobs are no longer an easy to reach thing. In fact it’s a daunting task to get the right job that you are looking for. So when you are applying for a pharma job make sure that your cover letter must be expressive enough. Highlight your area of interest and expertise, explain about your education and experience and relates it with the post you are applying for. Make the reader feels as only you are the right candidate for it.

Here is a sample pharma job cover letter:

From:



Ajay Prakash

S-10, LifeAround Avenue,

New Colony,

Colombia

To:

Mr. Roderick Tipton

Sprit Pharmaceuticals,

Trade Road,

Colombia

15th June, 2010

Dear Mr. Dubey,

This letter is in response to your advertisement of an opening in pharmacy research division posted on your company website. I found that my skills and experience match this position, so I would like to offer my services for this post.

I hold a professional degree in clinical pharmacy and have five years of work experience in the same field. Experimenting with biochemical compounds and discovering new medicines was my day to day work. Along with the following of set protocol, I made few new test methods which were very effective in diagnosis of the biochemical reactions that took place during the experiments.

I strongly believe that with the rapidly changing environment, need of good and authentic pharmacy research is in demand to produce immune friendly drugs. I can assure you that I will be an asset to your team and will bring the right and appropriate production in process.

I would appreciate the opportunity to have a personal meeting with you and looking for a call for the same.

Thank You,

Sincerely

Ajay Prakash

(241-877-6600)

Sunday, November 14, 2010

High performance liquid chromatography

High performance liquid chromatography
high performance liquid chromatography
.








High-performance liquid chromatography (or High pressure liquid chromatography, HPLC) is a form of column chromatography used frequently in biochemistry and analytical chemistry to separate, identify, and quantify compounds. HPLC utilizes a column that holds chromatographic packing material (stationary phase), a pump that moves the mobile phase(s) through the column, and a detector that shows the retention times of the molecules. Retention time varies depending on the interactions between the stationary phase, the molecules being analyzed, and the solvent(s) used.
Operation

the sample to be analyzed is introduced in small volume to the stream of mobile phase and is retarded by specific chemical or physical interactions with the stationary phase as it traverses the length of the column. The amount of retardation depends on the nature of the analyte, stationary phase and mobile phase composition. The time at which a specific analyte elutes (comes out of the end of the column) is called the retention time and is considered a reasonably unique identifying characteristic of a given analyte. The use of pressure increases the linear velocity (speed) giving the components less time to diffuse within the column, leading to improved resolution in the resulting chromatogram. Common solvents used include any miscible combinations of water or various organic liquids (the most common are methanol and acetonitrile). Water may contain buffers or salts to assist in the separation of the analyte components, or compounds such as trifluoroacetic acid which acts as an ion pairing agent.
A further refinement to HPLC has been to vary the mobile phase composition during the analysis; this is known as gradient elution. A normal gradient for reversed phase chromatography might start at 5% methanol and progress linearly to 50% methanol over 25 minutes, depending on how hydrophobic the analyte is. The gradient separates the analyte mixtures as a function of the affinity of the analyte for the current mobile phase composition relative to the stationary phase. This partitioning process is similar to that which occurs during a liquid-liquid extraction but is continuous, not step-wise. In this example, using a water/methanol gradient, the more hydrophobic components will elute (come off the column) under conditions of relatively high methanol (which is hydrophobic); whereas the more hydrophilic compounds will elute under conditions of relatively low methanol/high water. The choice of solvents, additives and gradient depend on the nature of the stationary phase and the analyte. Often a series of tests are performed on the analyte and a number of generic runs may be processed in order to find the HPLC method which gives the best separation of peaks.
Types of HPLC
Normal phase chromatography
Also known Normal phase HPLC (NP-HPLC), this method separates analytes based on polarity; it was the first kind of HPLC that chemists developed. NP-HPLC uses a polar stationary phase and a non-polar mobile phase, and works effectively for relatively polar analytes. The polar analyte associates with and is retained by the polar stationary phase. Adsorption strengths increase with increased analyte polarity, and the interaction between the polar analyte and the polar stationary phase (relative to the mobile phase) increases the elution time. The interaction strength depends not only on the functional groups in the analyte molecule, but also on steric factors. The effect of sterics on interaction strength allows this method to resolve (separate) structural isomers.
Use of more polar solvents in the mobile phase will decrease the retention time of the analytes, wheras more hydrophobic solvents tend to increase retention times. Very polar solvents in a mixture tend to deactivate the column by occupying the stationary phase surface. This is somewhat particular to normal phase because it is most purely an adsorptive mechanism (the interactions are with a hard surface rather than a soft layer on a surface).
NP-HPLC had fallen out of favor in the 1970s with the development of reversed-phase HPLC because of a lack of reproducibility of retention times as water or protic organic solvents changed the hydration state of the silica or alumina chromatographic media. Recently it has become useful again with the development of HILIC bonded phases which improve reproducibility.
Reversed phase chromatography
Reversed phase HPLC (RP-HPLC or RPC) has a non-polar stationary phase and an aqueous, moderately polar mobile phase. One common stationary phase is a silica which has been treated with RMe2SiCl, where R is a straight chain alkyl group such as C18H37 or C8H17. With these stationary phases, retention time is longer for molecules which are more non-polar, while polar molecules elute more readily. An investigator can also increase retention time by adding a polar solvent to the mobile phase, or decrease retention time by adding a more hydrophobic solvent. RPC is so commonly used that it is often incorrectly referred to as "HPLC" without further specification. The pharmaceutical industry regularly employs RPC to qualify drugs before their release.
RPC operates on the principle of hydrophobic interactions, which result from repulsive forces between a polar eluent, the relatively non-polar analyte, and the non-polar stationary phase. The binding of the analyte to the stationary phase is proportional to the contact surface area around the non-polar segment of the analyte molecule upon association with the ligand in the aqueous eluent. This solvophobic effect is dominated by the force of water for "cavity-reduction" around the analyte and the C18-chain versus the complex of both. The energy released in this process is proportional to the surface tension of the eluent (water: 7.3 × 10-6 J/cm², methanol: 2.2 × 10-6 J/cm²) and to the hydrophobic surface of the analyte and the ligand respectively. The retention can be decreased by adding a less polar solvent (methanol, acetonitrile) into the mobile phase to reduce the surface tension of water. Gradient elution uses this effect by automatically changing the polarity of the mobile phase during the course of the analysis.
Structural properties of the analyte molecule play an important role in its retention characteristics. In general, an analyte with a larger hydrophobic surface area (C-H, C-C, and generally non-polar atomic bonds, such as S-S and others) results in a longer retention time because it increases the molecule's non-polar surface area, which is non-interacting with the water structure. On the other hand, polar groups, such as -OH, -NH2, COO- or -NH3+ reduce retention as they are well integrated into water. Very large molecules, however, can result in an incomplete interaction between the large analyte surface and the ligand's alkyl chains and can have problems entering the pores of the stationary phase.
Retention time increases with hydrophobic (non-polar) surface area. Branched chain compounds elute more rapidly than their corresponding linear isomers because the overall surface area is decreased. Similarly organic compounds with single C-C-bonds elute later than those with a C=C or C-C-triple bond, as the double or triple bond is shorter than a single C-C-bond.
Aside from mobile phase surface tension (organizational strength in eluent structure), other mobile phase modifiers can affect analyte retention. For example, the addition of inorganic salts causes a moderate linear increase in the surface tension of aqueous solutions (ca. 1.5 × 10-7 J/cm² per Mol for NaCl, 2.5 × 10-7 J/cm² per Mol for (NH4)2SO4), and because the entropy of the analyte-solvent interface is controlled by surface tension, the addition of salts tend to increase the retention time. This technique is used for mild separation and recovery of proteins and protection of their biological activity in protein analysis (hydrophobic interaction chromatography, HIC).
Another important component is the influence of the pH since this can change the hydrophobicity of the analyte. For this reason most methods use a buffering agent, such as sodium phosphate, to control the pH. The buffers serve multiple purposes: they control pH, neutralize the charge on any residual exposed silica on the stationary phase and act as ion pairing agents to neutralize charge on the analyte. Ammonium formate is commonly added in mass spectrometry to improve detection of cerain analytes by the formation of ammonium adducts. A volatile organic acid such as acetic acid, or most commonly formic acid, is often added to the mobile phase if mass spectrometry is used to analyze the column eluent. Trifluoroacetic acid is used infrequently in mass spectrometry applications due to its persistence in the detector and solvent delivery system, but can be effective in improving retention of analytes such as carboxylic acids in applications utilizing other detectors, as it is one of the strongest organic acids. The effects of acids and buffers vary by application but generally improve the chromatography.
Reversed phase columns are quite difficult to damage compared with normal silica columns; however, many reversed phase columns consist of alkyl derivatized silica particles and should never be used with aqueous bases as these will destroy the underlying silica particle. They can be used with aqueous acid, but the column should not be exposed to the acid for too long, as it can corrode the metal parts of the HPLC equipment. RP-HPLC columns should be flushed with clean solvent after use to remove residual acids or buffers, and stored in an appropriate composition of solvent. The metal content of HPLC columns must be kept low if the best possible ability to separate substances is to be retained. A good test for the metal content of a column is to inject a sample which is a mixture of 2,2'- and 4,4'- bipyridine. Because the 2,2'-bipy can chelate the metal, the shape of the peak for the 2,2'-bipy will be distorted (tailed) when metal ions are present on the surface of the silica.[citation needed]..
Size exclusion chromatography
Size exclusion chromatography (SEC), also known as gel permeation chromatography or gel filtration chromatography, separates particles on the basis of size. It is generally a low resolution chromatography and thus it is often reserved for the final, "polishing" step of a purification. It is also useful for determining the tertiary structure and quaternary structure of purified proteins.
This technique is widely used for the molecular weight determination of polysaccharides. SEC is the official technique (suggested by European pharmacopeia) for the molecular weight comparison of different commercially available low-molecular weight heparins.
Ion exchange chromatography.
In ion-exchange chromatography, retention is based on the attraction between solute ions and charged sites bound to the stationary phase. Ions of the same charge are excluded. Types of ion exchangers include:
• Polystyrene resins – These allow cross linkage which increases the stability of the chain. Higher cross linkage reduces swerving, which increases the equilibration time and ultimately improves selectivity.
• Cellulose and dextran ion exchangers (gels) – These possess larger pore sizes and low charge densities making them suitable for protein separation.
• Controlled-pore glass or porous silica
In general, ion exchangers favor the binding of ions of higher charge and smaller radius.
An increase in counter ion (with respect to the functional groups in resins) concentration reduces the retention time. An increase in pH reduces the retention time in cation exchange while a decrease in pH reduces the retention time in anion exchange.
This form of chromatography is widely used in the following applications: water purification, preconcentration of trace components, ligand-exchange chromatography, ion-exchange chromatography of proteins, high-pH anion-exchange chromatography of carbohydrates and oligosaccharides, and others.
Bioaffinity chromatography
This chromatographic process relies on the property of biologically active substances to form stable, specific, and reversible complexes. The formation of these complexes involves the participation of common molecular forces such as the Van der Waals interaction, electrostatic interaction, dipole-dipole interaction, hydrophobic interaction, and the hydrogen bond. An efficient, biospecific bond is formed by a simultaneous and concerted action of several of these forces in the complementary binding sites.
Isocratic flow and gradient elution
A composition of the mobile phase that remains constant throughout the procedure is termed isocratic.
In contrast to this is the so called "gradient elution", which is a separation where the mobile phase changes its composition during a separation process. One example is a gradient in 20 min starting from 10% methanol and ending up with 30% methanol. Such a gradient can be increasing or decreasing. The benefit of gradient elution is that it helps speed up elution by allowing components that elute more quickly to come off the column under different conditions than components which are more readily retained by the column. By changing the composition of the solvent, components that are to be resolved can be selectively more or less associated with the mobile phase. As a result, at equilibrium they spend more time in the solvent and less time in the stationary phase, and therefore they elute faster.
Parameters
Internal diameter
The internal diameter (ID) of an HPLC column is a critical aspect that influences sensitivity and determines the quantity of analyte that can be loaded onto the column. Larger columns are usually seen in industrial applications such as the purification of a drug product for later use. Low ID columns have improved sensitivity and lower solvent consumption at the expense of loading capacity.
• Larger ID columns (over 10 mm) are used to purify usable amounts of material because of their large loading capacity.
• Analytical scale columns (4.6 mm) have been the most common type of columns, though smaller columns are rapidly gaining in popularity. They are used in traditional quantitative analysis of samples and often use a UV-Vis absorbance detector.
• Narrow-bore columns (1-2 mm) are used for applications when more sensitivity is desired either with special UV-vis detectors, fluorescence detection or with other detection methods like liquid chromatography-mass spectrometry
• Capillary columns (under 0.3 mm) are used almost exclusively with alternative detection means such as mass spectrometry. They are usually made from fused silica capillaries, rather than the stainless steel tubing that larger columns employ.
Particle size
Most traditional HPLC is performed with the stationary phase attached to the outside of small spherical silica particles (very small beads). These particles come in a variety of sizes with 5 μm beads being the most common. Smaller particles generally provide more surface area and better separations, but the pressure required for optimum linear velocity increases by the inverse of the particle diameter squared.[1][2][3]
This means that changing to particles that are half as big, keeping the size of the column the same, will double the performance, but increase the required pressure by a factor of four. Larger particles are used in preparative HPLC (column diameters 5 cm up to >30 cm) and for non-HPLC applications such as solid-phase extraction.
Pore size
Many stationary phases are porous to provide greater surface area. Small pores provide greater surface area while larger pore size has better kinetics, especially for larger analytes. For example, a protein which is only slightly smaller than a pore might enter the pore but not easily leave once inside.
Pump pressure
Pumps vary in pressure capacity, but their performance is measured on their ability to yield a consistent and reproducible flow rate. Pressure may reach as high as 40 MPa (6000 lbf/in2), or about 400 atmospheres. Modern HPLC systems have been improved to work at much higher pressures, and therefore are able to use much smaller particle sizes in the columns (<2 μm). These "Ultra High Performance Liquid Chromatography" systems or UHPLCs can work at up to 100 MPa (15,000 lbf/in²), or about 1000 atmospheres. The term "UPLC", though sometimes used is a trademark of Waters Corporation and not the name for the technique in general.